Welcome, JR!
Things like "T205" and "E90" are set designations from the American Card Catalog, which went through several editions between 1939 and 1960, and is still widely used among collectors of pre-WWII cards. There are various places where you can learn more about these designations, but one of the best for a beginner is on the Old Cardboard site:
http://oldcardboard.com/ref/classifications.asp
The rest of that site has lots of good information about all these different sets, and about vintage cards in general.