Friendship

Friendship is common both in The Sims and The Sims 2. When two Sims' (daily) relationships are over 50, the Sims become friends.

Becoming friends with another Sim allows greater social interactions, which are less likely to be rejected by the receiving Sim. This means that gaining a higher status relationship (Best Friends, Crush, etc.) is easier to achieve.

Friendships are slightly different in The Sims 3 since there is only one friendship bar not two (short and long term) and your friendship status changes as you move up the bar (from stranger to acquaintence, to friend, to good friend, to best friend