Murder is most commonly the crime of intentionally causing the death of another human being without lawful excuse. Where the death was not caused intentionally, but was caused by negligence or recklessness, the crime committed is manslaughter, which is less serious than murder.
A difficult issue in defining murder is what counts as causing death. It is impossible to give a precise definition of this, but some legal principles have been developed to help. For example, many common law jurisdictions abide by the year and a day rule, which provides that one is to be held responsible for a persons death only if they die within a year and a day of the act. Thus, if you seriously injured someone, and they died from their injuries within a year and a day, you would be guilty of murder; but you would not be guilty if they died from their injuries after a year and a day had passed.
It is not murder to kill someone with lawful excuse; lawful excuses include killing in time of war, killing a person who poses an immediate threat to the lives of ones self or others (i.e., in self-defence), and executing a person in accordance with a sentence of death (in those jurisdictions which retain capital punishment). Sometimes extreme provocation or duress can justify killing another as well. These cases of killing are called justifiable homicide.
Under British law (and the law of other countries, such as Australia, which pay close heed to the decisions of British courts), it is murder to kill another human being for food, even if without doing so one would die of starvation. This originated in a case of three shipwrecked sailors cast adrift off the coast of South Africa in the 1920s; two of the sailors conspired to kill the other sailor, and having killed him ate his flesh to survive.
Many jurisdictions in the United States have adopted felony murder statutes, according to which anyone who commits a serious crime (a felony), which results in a person's death, is guilty of murder. This applies even if one is not actually responsible for the person who died; for example, a driver for an armed robbery can be convicted of murder if one of the robbers killed someone in the process of the robbery, even though the driver was not (directly, at least) responsible for this person's death.