Divorce

Contrary to popular belief, it is possible for marriages in the Sims' world to end in ways other than death.

If two married Sims have their relationship with each other fall enough, such as through witnessing an affair one Sim can kick the other to the curb. To have the couple get a divorce, choose whichever Sim you want to do the breaking up, click the other spouse, and click "Break Up". This option will only appear if the active Sim's relationship is low enough with their spouse. The divorcee will then flee the lot and reappear in the Family bin in the neighborhood screen. They have no chance to take any children they have with them or remain in the house. If you want them to stay in the house living with their family after the divorce, you will have to move them back in.

Avoiding Divorce
If you want your Sims to avoid divorce you can either wait for them to stop being mad at each other or with apartment life you can have them do a interaction called: Repair Relationship with... which will have the two Sims try and deal with their problems.