They were a mechanical centerpiece to the Phyrexian faction in both blocks
Its most distinctive feature (and design faux pas) is that it doesn't have a design on the back of the card just a blank white canvas
All the good cards contribute to infinite combos, and theres like almost 20 cards that do that, so theres no real way to play a decent energy deck without that as a possibility
Keep track of your collection value over time
We've seen good examples of this over the past couple of summers, where the summer creates a hole in the schedule for those who aren't interested in Limited formats of the summer set or in Alchemy
Aside from battling, you can also trade cards (it is a trading card game, after all) with users through an online trades marketplace, where you can put cards up for offer and browse cards other users are offering