Sorin Markov is a planeswalker who's effects need loyalty counters to use. The number of loyalty counters also means how much damage he can take before he is destroyed. You cannot have more than one Planeswalker-Sorin on the field at once. Planeswalkers are not Creatures, so can not be targetted by spells saying "target creature" and are unaffected by other spells affecting creatures, such as Wrath of God and Mass Calcify.
Sorin is probably one of the most useful planeswalkers to date. His abilities are none shorter than short and blunt. His first ability is a lose life/gain life ability, which in itself it decent, but nothing really surprising. His 2nd ability is pretty good. Being able to reset an opponents life total to 10 can stop life gain and stall decks in their paths, but as usual, it's the last ability that shines. By taking away 7 counters, he can control an opponents next turn. Not many cards in the Magic The Gathering universe can do this. They'll basically play what you want to play. You get to see their hand and all!