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). A friendship's status changes as the bar increases or decreases (from stranger to acquaintance, to friend, to good friend, to best friend - and into romance).

Friendships do still decay but the decay seems to be slower. It is difficult to tell since there is no longer a number to look at, just a bar that fills up.

In The Sims 3, if a relationship decays below friendship, the other Sim now becomes a "distant friend", so they aren't lost as a friend. It is unknown if the friendship can decay below that level, or what it becomes if it does.