user25568: Want to know commands (codes) for following formatting tasks. 

Degree Centigrade.
Subscripts and superscripts like CO2, 14N , (6C13)6H12O6. 
Greek letters.
a portion of text, with special characters like stars, less-than, greater-than etc, that would not considered as a part of command (like nowiki command in wikipedia). 
Displaying more-than 1 space-bars between 2 letters.

Also want to know, what-type of programming language? Surely it is not Wikicode. Is there any more-detail guidelines to handle this code?
Look how strange things a user has done on this website. Stack Exchange experimentation sandbox A big salute to that user.

user25568: I've found following things useful. 
1. As a general-purpose, rapid, short command for subscript and superscript: 
<sub>subscript</sub> and <sup>superscript</sup> in the edit field; gives rise to respectively subscript and superscript . 
So; for example;

to write Cl2 ; type Cl<sub>2</sub> .
To write 23 ; type 2<sup>3</sup>
To write 6C12 or 126C ; respectively type  <sub>6</sub>C<sup>12</sup> or <sup>12</sup><sub>6</sub>C. 
To write 12C6H12O6 or  13C*6H12O6, respectively;    type  <sup>12</sup>C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> or <sup>13</sup>C<sup>*</sup><sub>6</sub>H<sub>12</sub>O<sub>6</sub>.


Source and Courtesy: User @Terdon.


2. To type something that will not be treated as command:
there are 2 ways:


A. 4-successive space bars before the text.


But that adds a horizontal scroll-bar. 


B. Quoting the text by covering it inside 2 backtick marks. This could be done inside sentence;  



Source: seen while trying to understand the tools at visual edit toolbar. 



P.S. At help page there is an article about correct formatting. Beginners need to go through it. I didn't knew about it and I took many days to search it out.
There is also a page on comment-formatting


user948: There are two ways to do what you want: HTML Entities and MathJax. You can use HTML entities for characters like:

° &deg;
α &alpha;
Α &Alpha; (note the capitalization)
λ &lambda;

etc.

MathJax is basically a JavaScript implementation of $\LaTeX$ (that's $\LaTeX$ for those who are taking notes), a text formatting language. $\LaTeX$ itself is gigantic, so MathJax aims to implement a limited subset (still quite large) related to mathematics and other technical disciplines like chemistry. Rather than repeat a whole bunch of data presented elsewhere, here are some links:

MathJax basic tutorial and quick reference from the Math.SE meta site.
How can I format math/chemistry expressions here? from the Chemistry.SE meta site.
An online $\LaTeX$ Equation Editor

Finally, one quick hint that'll help you learn - if you come across something formatted in $\LaTeX$ on a Stack Exchange site (and possibly elsewhere online where they're using MathJax), just right-click on it and select Show Math As → TeX Commands and a popup will open showing the commands used, instead of having to edit the question/answer to see the source. Please also note you cannot use HTML Entities in MathJax, but there are symbols/commands for everything you'll need already built-in.
There are many $\LaTeX$/MathJax resources online, so if you get stuck, just Google.

25568: <p>Want to know commands (codes) for following formatting tasks. </p>

<ol>
<li><p>Degree Centigrade.</p></li>
<li><p>Subscripts and superscripts like CO2, 14N , (6C13)6H12O6. </p></li>
<li><p>Greek letters.</p></li>
<li><p>a portion of text, with special characters like stars, less-than, greater-than etc, that would not considered as a part of command (like nowiki command in wikipedia). </p></li>
<li><p>Displaying more-than 1 space-bars between 2 letters.</p></li>
</ol>

<p>Also want to know, what-type of programming language? Surely it is not Wikicode. Is there any more-detail guidelines to handle this code?</p>

<p>Look how strange things a user has done on this website. <a href="https://biology.meta.stackexchange.com/questions/654/stack-exchange-experimentation-sandbox">Stack Exchange experimentation sandbox</a> A big salute to that user.</p>
 25568: <p>I've found following things useful. </p>

<p><strong>1. As a general-purpose, rapid, short command for subscript and superscript:</strong>  </p>

<p><code>&lt;sub&gt;subscript&lt;/sub&gt;</code> and <code>&lt;sup&gt;superscript&lt;/sup&gt;</code> in the edit field; gives rise to respectively <sub>subscript</sub> and <sup>superscript</sup> . </p>

<p>So; for example;</p>

<ul>
<li><p>to write Cl<sub>2</sub> ; type <code>Cl&lt;sub&gt;2&lt;/sub&gt;</code> .</p></li>
<li><p>To write 2<sup>3</sup> ; type <code>2&lt;sup&gt;3&lt;/sup&gt;</code></p></li>
<li><p>To write <sub>6</sub>C<sup>12</sup> or <sup>12</sup><sub>6</sub>C ; respectively type  <code>&lt;sub&gt;6&lt;/sub&gt;C&lt;sup&gt;12&lt;/sup&gt;</code> or <code>&lt;sup&gt;12&lt;/sup&gt;&lt;sub&gt;6&lt;/sub&gt;C</code>. </p></li>
<li><p>To write <sup>12</sup>C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> or  <sup>13</sup>C<sup>*</sup><sub>6</sub>H<sub>12</sub>O<sub>6</sub>, respectively;    type  <code>&lt;sup&gt;12&lt;/sup&gt;C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;</code> or <code>&lt;sup&gt;13&lt;/sup&gt;C&lt;sup&gt;*&lt;/sup&gt;&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;</code>.</p></li>
</ul>

<blockquote>
  <p>Source and Courtesy: User @Terdon.</p>
</blockquote>

<hr>

<p><strong>2. To type something that will not be treated as command:</strong></p>

<p>there are 2 ways:</p>

<ul>
<li><ol start="2">
<li>A. 4-successive space bars before the text.</li>
</ol></li>
</ul>

<p>But that adds a horizontal scroll-bar. </p>

<ul>
<li><ol start="2">
<li>B. Quoting the text by covering it inside <em>2 <a href="http://www.thefreedictionary.com/Backtick" rel="nofollow noreferrer">backtick</a> marks</em>. This could be done inside sentence;  </li>
</ol></li>
</ul>

<blockquote>
  <p>Source: seen while trying to understand the tools at visual edit toolbar. </p>
</blockquote>

<hr>

<ul>
<li><p><em>P.S. At <a href="https://biology.stackexchange.com/help">help page</a> there is an article about correct <a href="https://biology.stackexchange.com/help/formatting">formatting</a>. Beginners need to go through it. I didn't knew about it and I took many days to search it out.</em></p></li>
<li><p><em>There is also a page on <a href="https://biology.stackexchange.com/editing-help#comment-formatting">comment-formatting</em></a></p></li>
</ul>
 948: <p>There are two ways to do what you want: <a href="https://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references" rel="nofollow noreferrer">HTML Entities</a> and <a href="http://www.mathjax.org" rel="nofollow noreferrer">MathJax</a>. You can use HTML entities for characters like:</p>

<ul>
<li>&deg; <code>&amp;deg;</code></li>
<li>&alpha; <code>&amp;alpha;</code></li>
<li>&Alpha; <code>&amp;Alpha;</code> (note the capitalization)</li>
<li>&lambda; <code>&amp;lambda;</code></li>
</ul>

<p>etc.</p>

<hr>

<p>MathJax is basically a JavaScript implementation of <a href="https://www.latex-project.org/" rel="nofollow noreferrer">$\LaTeX$</a> (that's <code>$\LaTeX$</code> for those who are taking notes), a text formatting language. $\LaTeX$ itself is gigantic, so MathJax aims to implement a limited subset (still quite large) related to mathematics and other technical disciplines like chemistry. Rather than repeat a whole bunch of data presented elsewhere, here are some links:</p>

<ul>
<li><a href="https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference">MathJax basic tutorial and quick reference</a> from the Math.SE meta site.</li>
<li><a href="https://chemistry.meta.stackexchange.com/questions/86/how-can-i-format-math-chemistry-expressions-here">How can I format math/chemistry expressions here?</a> from the Chemistry.SE meta site.</li>
<li><a href="http://www.codecogs.com/latex/eqneditor.php" rel="nofollow noreferrer">An online $\LaTeX$ Equation Editor</a></li>
</ul>

<p>Finally, one quick hint that'll help you learn - if you come across something formatted in $\LaTeX$ on a Stack Exchange site (and possibly elsewhere online where they're using MathJax), just right-click on it and select <strong><code>Show Math As → TeX Commands</code></strong> and a popup will open showing the commands used, instead of having to edit the question/answer to see the source. Please also note you cannot use HTML Entities in MathJax, but there are symbols/commands for everything you'll need already built-in.</p>

<p>There are <em>many</em> $\LaTeX$/MathJax resources online, so if you get stuck, just Google.</p>
