CS:GO (Counter-Strike: Global Offensive) is a 5v5 tactical shooter. First team to win 16 rounds wins the game. One team starts as terrorists (T's) and the others as counter-terrorists (CT's). The T's have 1:55min time to kill all CT's or to plant a bomb at one of two bomb sites. If the bomb is planted, it will explode after 40s if the CT's can't defuse it. After 15 rounds, T's and CT's switch sides.
Yes, since 2019, CS:GO is free-to-play on Steam. You can 15$ for "prime status" in regular matchmaking but it's not really neccessary because most players play on FACEIT because of better experience due better servers, an anti-cheat and a more competitive playerbase in general.
You can download CS:GO on Steam for FREE. The game's size is 28 GB and it runs even on low-spec computers. You can find the minimum requirements there too.