There are 3 basic sleeping bag styles. There is no real performance difference between them. The choice is purely a matter of personal preference.

Envelope is the term for the traditional 'square' type of sleeping bag. Great for general purpose camping, hiking, caravaning etc. Loads of foot room, but usually lacks a hood.

Mummy sleeping bags were mainly developed with backpackers in mind, for use where pack size and weight is important. By tapering the sleeping bag to fit the body all the 'non-important' parts of the bag have been left out, thus reducing pack size and weight. Great when the sleeping bag has to be carried in a rucksack.

Mummy style sleeping bags also have a hood which can be drawn in so that only the face is exposed. This traps the warm air inside the sleeping bag more effectively - not to mention keeping your ears warm!

Some people feel uncomfortable in a Mummy style sleeping bag. The tapering towards the feet can make it feel as if you have your legs tied together! A hybrid sleeping bag, on the other hand, has the hood of a Mummy bag with the square end of an Envelope sleeping bag. The best of both worlds!