system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The difference between GSM, CDMA, 3G, 4G, LTE, etc. 

I know GSM is what ATT/T-Mobile use and CDMA is Verizon/Sprint, but that's about the extent of my knowledge. I'd actually prefer an explanation like I'm 21 and not an engineer. Thanks!
gpt
To things are needed for a cellular network:

1) How to get many people to share a piece of spectrum.

2) How to send data from cell towers to phones and vice-versa.

GSM and CDMA are two different ways to accomplish the two things.  LTE is newer.

The way GSM solves (1) is by something called TDMA (time division multiple access).  When you're in a phone call, you're phone is scheduled a bunch of time slots when your phone either sends or receives data.  These exclusive to your phone and different from other phones in the cell so there's no interference.  This way, multiple phones can talk to the cell tower (seemingly) at once (the bursts of time are super short so you don't notice them).

CDMA deals with (1) in a completely different way.  It breaks up the channel into codes/signals (Code division random access).  This is a little hard to explain without some math, but there's a notion called orthogonality.  If two signals are orthogonal you can pull one signal out without getting interference from the other.  Every user is assigned a different code/signal and these are (approximately) orthogonal to each other.  This is a more advanced technique and generally thought of as advantageous since there isn't as much waste (TDMA needs little bits of extra time between users to make sure there's no overlap, for example).

The way (2) is accomplished is also very different.  In fact there are many different ways it is done even within GSM or CDMA.  The way data is sent along depends a lot on how good the quality of the radio signal and other factors.  That's a whole other thing.  But the options for GSM and CDMA differ.

3G and 4G are kind of marketing terms that come from "3rd generation" and "4th generation".  They refer to families of standards, but not specific methods to accomplish (1) or (2).

LTE is a 4G standard that uses OFDM to do (1).  This is kind of like TDMA but instead of time, it's in frequency.  But they do some clever trickery to make it work really without needing gaps between users' frequencies.

I can explain more, if you'd like, but it gets really technical really quick.