Definition of buy-in
The buy-in is the amount you pay to get your starting chips in a tournament or sit'n'go. The buy-in will go into the main prize pool to make up the paid seats. The total buy-in also includes the rake or commission that the cardroom takes. A tournament with a 100+10 buy-in is a tournament with a 110 Euro buy-in. 10 will go to the cardroom and 100 to the prize pool. This money will be converted into chips. If you play online poker, most of the time in a tournament, several different buy-ins will give you the same number of starting chips. It is the amount of the payouts that will be different.
Buy-in in cash games
We can also talk about buy-ins in cash games. At a table with blinds of 1 euro and 2 euros, the maximum buy-in is often 100 times the big blind, i.e. 200 euros. You can also buy in for a minimum of 20 big blinds. You can say: "I have a buy-in for 20 big blinds at this table" or "I have a buy-in for 100 big blinds at this table".