They are all used for Texas Hold'em and, perhaps, some other poker games.

"Overs" buttons are used when the game has a structure, i.e. 4-4-8-8, but some players want to play pot limit. Those who want to play pot limit take an "Overs" button and put it on the table next to their chips. When the only people left in a hand have "Overs" buttons, the hand changes from structured to pot limit. Pot limit means that a player can bet any amount up to the amount in the pot.

"1/2K" is a half kill button. In some games, a "kill" takes place when the same player wins two hands in a row. In other games, a "kill" takes place when the pot reaches a predetermined amount, say $40 for a 4-8 game. The next hand is a "kill" hand. A "full kill" means the structure doubles--i.e. if the game is 4-8, the "kill" hand becomes 8-16. A "half kill" means that the next hand is 1.5 times the original structure so for 4-8 it becomes 6-12.

Probably more than you wanted to know---

Don