Tumbler= is basically a pot that vibrates, inside you put your tarnished brass and media (crushed up corn cob, walnut shells etc) and let it polish the brass for about 2-3 hours

Trimmer is actually a case trimmer, it is a mini lathe that has a chuck on one end, and a 4 fluted cutter on the other, you chuck up your casing that has expanded in length due to firing, and "trim" it back to original length. (Its strictly a safety step, but very much needed)

Dies are a tool that is screwed into the top of your press. You have 2 different dies a resizing die that is used to resize the case which has expanded in the firing process back to the original size. And your seating die which is used to seat the bullet into your brass once it has returned to original size.

these are very basic definitions and many different varieties of these exist depending on ones preference and wallet. For example, you can buy a trimmer that is actuated with a small motor instead of cranking it by hand, you can use electronic scales or beam scales, it all depends on your wallet and how deep you want to get into this adicting habit. I would say to start out get the kit from RCBS that has the Rock crusher in it. I love mine, the only thing I bought extra was a tumbler, and an extra shell tray, it only comes with one, and I find at least 2 are needed. I would recommend to get a good set of verneers (calipers) that measures down to .001. And last a few manuals especially one from the manufacturer of the bullets you intend to shoot.