Core Game modes form the base game modes of Critical Ops, and they are always available.

In Defuse, the Breach team tries to plant the bomb and defend it until it detonates. While the Coalition team tries to stop them.
The round ends when the bomb has detonated or has been defused.
You can also win by eliminating the enemy team before the bomb has been planted. The team that reaches 9 round victories first, wins the match.

Team Deathmatch
Opposing teams keep killing each other until the timer runs out. The team with the most kills wins.

Whoever goes down, stays down.  Last team standing wins!

Practice Mode
Practice mode allows you to host a Custom Game that includes bot teammates and opponents.
To start a Practice game, go to Play => Custom Games => Host and change the game type to ‘Practice’