The divorce petition will contain specific claims and statements of fact that are alleged to justify the divorce. You must specifically answer each of these claims and statements fact, and either admit or deny each one. For example, your spouse may claim to be entitled to sole physical custody of the children. You may deny this claim and provide supporting statements of fact, such as the fact that your children would have to change schools to live with your spouse.
You are entitled to make counterclaims -- claims that you would have made if you had filed the petition yourself. For example, you may demand sole physical custody of the children, or you may ask the court to assess all court costs and legal fees against your spouse. Your counterclaims should be accompanied by statements of fact that, if proven true, would justify the demands that you are making. If you and your spouse already made a marital dissolution agreement and his petition reflects the terms of the agreement, you may simply admit all of his claims and statements of fact and decline to make any counterclaims.
Different jurisdictions impose different formal requirements for an answer. Although some jurisdictions require you to draft your answer on your own, many require the use of pre-printed forms. For example, each of your spouse's claims and statements of fact may be numbered, and you may be required to check "admitted" or "denied" and provide a brief explanation. The form may include a separate section for counterclaims. It will also include a certificate of service in which you confirm that you served a copy on your spouse or his attorney. It may also include an identify verification section requiring your notarized signature.
If you fail to file your answer by the deadline, or if you send it by regular mail and it never arrives, the court may allow you to file a late answer by signing a form as long as you show up at the hearing. If you don't file an answer and don't show up at the hearing, the court may enter a default judgment against you, and the terms of the divorce may reflect terms demanded by your spouse. If a default judgment is entered against you, you may contest it later if you can show that you were never served with the summons. You may also contest child support and custody orders, even if you ignored the summons, by showing that these orders are not in the best interests of the child based on evidence that was not presented at the hearing.