ilovemissyelliott: a tank is a pokemon that can survive lots of attacks (often specializing in special or physical attacks). an example
blissey@leftovers
softboiled
thunderbolt
toxic
heal bell
probably not a good set, but i'm too lazy to look up the standard. anyway, blissey's sp def is insanely high and her hp is REALLY insanely high, which basically means mewtwo can shove six or seven psychics at blissey and she won't faint. tanks sometimes don't have very many attacking moves, sometimes none at all (common on umbreon, whose attacking movepool is extremely limited). other times you have tanks that are heavy hitters as well as tanks - take donphan for instance.
sawe: you aren't very specific about your purposes, and the tank you'd need really depends on your team. blissey and umbreon are great special tanks and skarmory and donphan are quite good physically.
it's been a while since i've played competitively, so forgive me for stupid mistakes :p
DDR: want to pass Paranoia Survivor ~ Religions are just cults with more members. -- The resident SSB:M Captain Falcon user. -- "Let's eat fast so we can eat again!" ~ Ash