In law, murder is commonly separated in two general types, first- and second-degree, related to the severity of the crime. A murder is treated as second degree by default, and certain conditions may change its status to first degree if a given number of them is met. In a broad sense, this is used to describe either severe intentions or severe method of execution of the murder, and first degree murder is punished more harshly by law in almost all jurisdictions.
In order for a murder to qualify as first degree, it must either have been the result of an aggravated assault (as opposed to accidental manslaughter or regular assault), or have been pre-meditated and planned extensively before occurring. This is why one of the most common defenses when charged with first degree murder is to claim that it was decided on the spot and had not been premeditated in any way, though this can usually easily be proven false by careful prosecutors.
Unusual methods of execution, such as using poison or torture, are also considered to be first degree murder, and may additionally be prosecuted with other charges, depending on the specific circumstances surrounding the crime. A first degree murder normally results in a severe extension of the punishment issued by court, and in some jurisdictions it's even eligible for capital punishment. Even though some jurisdictions do not recognize first degree murder by default, there are still elements in their laws which are used to distinguish between murders of varying severity