Is it okay to use a different edition of cards or "block" of cards against an opponent in magic the gathering?

Question

for example, if i have 10th edition cards, can i play someone with 8th edition cards, is that legal? or if i have shadowmoor and someone has onslaught, can we legally play against each other. ive read some rules on tournaments and its confusing, do you have to buy like every set or edition so you can face anyone? sorry, im brand new to the game, and i dont know much as far as this stuff. i dont wanna go play someone at a shop and they laugh in my face because we have two different editions or "blocks" of decks.

Answer 1

In Vintage or Legacy or Casual, yes.

In Specific Blocks, must be same block cards. (Like Onslaught deck vs. Scrouge, OK, Ice Age vs. Shadowmoor, big no-no)

In Type two the two most recent blocks and most recent Core set(tenth ed.) are legal.

When Eventide is released T2

(Time Spiral, Planar Choas, Future Sight, Coldsnap), (Tenth), (Lorwyn, Morningtide, Shadowmoor, Eventide)

When Shards of Alara Block T2

(Tenth), (Lorwyn, Morningtide, Shadowmoor, Eventide), (Shards of Alara, Paper, Scissors)

When Live comes out T2

(Shards of Alara, Paper, Scissors), (Eleventh), (Live, Long, Prosper)

and so on

Paper, Scissors, Live, Long, and Prosper are codenames, not real set names.

Help at all?