system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is plasma? How is it considered a fourth phase of matter?

As a followup question, is fire plasma?
gpt
Suppose you start with an ice cube.

In a solid, all the atoms are packed pretty rigidly together.  They vibrate around but don't move much.  In your ice cube, the atoms actually form a lattice, a repeating pattern.  Not all solids do this (amorphous solids have no repeating pattern).  Solids have a definite shape and volume.  If you put your ice cube on a table, in a room cold enough that it doesn't melt, it'll just sit there.  It keeps its shape and it doesn't change size.

If you add some energy to your ice cube, you can melt it.  Then you have a liquid.

In a liquid, the atoms have much more freedom of motion.  They still like to stick close to each other, but they slip and slide around.  Liquids have a definite volume (like solids) but no definite shape (this is new) - they take the shape of their container.  Melt your ice cube on that table and the water will spread out, attempting to cover the whole table in a flat puddle.  Unless you have a very small table, or an impressively large ice cube, it won't be able to.  Liquids do not change their volume.  Put that same ice cube in a cup and melt it, and the water will conform to the shape of the cup (and be a cylinder, instead of the flat puddle on the table).

Take that puddle of water and add even more energy.  You'll vaporize it.  Now you'll have a gas.

In a gas, atoms have even more freedom of motion.  They also no longer feel the need to stick close to each other.  Gases have neither definite shape (like liquids) nor definite volume (this is new) - they will expand to fill their container.  So that puddle of water on your table, in gas form, now spreads out to boundaries of the whole room (it's important to note the definition of volume, though.  Throughout all this melting, the number of atoms hasn't changed.  So if you cooled the room back down, it wouldn't be filled with ice - you'd have the same size ice cube as you started with, if you somehow were able to shepherd all the gaseous atoms back together.  What has happened is that the water vapor has spread out across the whole room, instead of staying concentrated in one part of the room like the ice cube).

Now take that water vapor and add even more energy, enough to ionize it.

Now you finally have plasma.  A plasma is similar to a gas, but even more free.  In a plasma, you have ionized the atoms of the gas.  Now instead of being a cloud of atoms, you have a cloud of positively and negatively charged particles (you broke the atoms apart into electrons and chunks of the nucleus.  The exact size and composition of these chunks depends on the substance and how much energy you added).  Since plasmas are composed of charged particles (which are called ions, which is why the process of making them is called ionization), they'll react to an externally applied electric or magnetic field.  Exactly how they react depends on the nature of the field you apply and some other things.  Plasmas have no definite shape or volume (like gases), but can form structures when subjected to certain fields (this is new - if you google plasma globes you'll see examples of filamentation, one of the shapes a plasma can be induced to take).

At each stage of the game here we have added energy and drastically altered the nature of the substance we were working with.  This is how we distinguish the phases of matter, and how plasma is the fourth.  We went from atoms packed rigidly together, to atoms that slip and slide past each other but don't get too far away, to atoms that just fly around anywhere, to atoms that have been broken up into smaller bits.  You can, in fact, keep adding energy and get to other, more exotic states of matter (if you add enough energy, you can break apart the neutrons and protons in the nuclei and get quark-gluon plasma.  Today this only happens in huge particle accelerators for brief instants, because the amount of energy required is.....very big.  But as the universe was cooling down after the Big Bang, there was a time where all matter was this).  And conversely, you can take energy away from solids (cool them down) and get more phases (these are the Bose-Einstein condensates.  In these, all the atoms kind of lump together into one super atom and you get a variety of quantum effects usually only seen with much smaller particles).  Then there are theoretical phases that haven't been observed yet, like color superconductors and all kinds of neat stuff.  

The bottom line is, a phase is a general state of matter where it behaves in a certain way.  Solids, liquids, and gases are the three phases that a person is most likely to run into in everyday life.  Plasma is another state of matter, often called the fourth state of matter because it is also not that hard to observe in basic settings.  Plasma is characterized by the ionization of constituent particles, and otherwise behaves much like a gas.

To answer your followup, yes.  Well, depending on whether or not you are a chemist either yes, or no, fire is a chemical reaction but flames are a plasma.  When you burn something, you are releasing a lot of energy, and turning whatever you burned into a gas.  If the fire is hot enough, the gas will ionize and you have yourself a plasma (where 'hot enough' is defined relative to what you are burning.  Different things ionize at different temperatures).