A premarital agreement (or prenup) is a contract between the two parties getting married. It can cover a number of different subjects, including spousal support and property division. It can be as creative or restrictive as the parties prefer. There are very few exceptions as to the topics that parties may contract over (one exception is child support). The goal is to protect each party's legal rights and obligations, and to make clear what the parties' intents are before marriage. Premarital agreements are also very helpful in situations with blended families, especially when the parties want to keep certain things intact for kids from a prior marriage or relationship.
The most common excuse for not getting a premarital agreement is that it's not romantic. There is nothing particularly romantic about discussing debts, property rights, and money. People often complain that having a premarital agreement drafted is tantamount to assuming that a divorce will be necessary one day. However, while it certainly is not romantic, it may be a logical choice for the parties to reach an agreement before getting married. It is an opportunity to ensure that the parties discuss important issues such as money or how property will be obtained during the marriage. It is also an opportunity for parties to define how they would like to divide property in the future, if they do not wish to follow the community property laws of California.
After getting married, the parties are still free to reach an agreement. People commonly have postmarital agreements drafted to address the purchase of real estate after getting married. It is a bit trickier to deal with these issues after marriage, especially when considering "transmutations", so we encourage you to seek legal counsel if you think you need a postmarital agreement.