I am definitely not trying to be "Captain Obvious" here, but have you visited the Festool page for Systainers?
http://www.festoolusa.com/power-tools/storage-systems/systainers/
I mention it because there are a lot of nice pictures and some good information to read as well. I understand that it can get a bit overwhelming - and it's difficult to determine scale - so let me try to add to what others have posted.
Classic systainers are the older style, and are available in 'regular' sizes of SYS1 - Sys5, and 'mini-systainers' that will fit inside (but not attach to) the SYS1-5. They are compatible with the newer T-LOC's with some caveats. You'll probably want to buy all T-LOC's since you're just starting out and have no classics to worry about.
T-LOC systainers are a newer style, and are available in 'regular' sizes of SYS1 - Sys5, and 'mini-systainers' that will fit inside (but not attach to) the SYS1-5.
(Just to make things MORE confusing, the quantity of mini-systainers that can fit in the SYS1-SYS5 *MAY BE* different for the new T-LOC style than it was for the old-style systainers. I'll let other address that because I can't speak from personal experience.)
Midi and Maxi Systainers are kinda special cases. The Maxi should be obvious as the largest size. The Midi should NOW be obvious as a size that is larger than the SYS1-SYS5, but smaller than the Maxi.
Finally, there are T-LOC Midi's that link up to the T-LOC Sys1-5. There is no T-LOC Maxi.
Does any of that help at all?