A register is a group of notes which have the same tonal characteristics. The singing voice is commonly divided into the chest register (or chest voice) and the head register (or head voice). When you sing a low note, you can feel the vibrations in your chest and throat.
When you sing a high note, you feel the vibration in the roof of your mouth and nasal area, as well as your head. When singing notes in the middle of your range, you use a combination of chest voice and head voice. Some people call this another register, while others see it as a blend of the two basic registers. One of the most difficult aspects of training the voice is getting a smooth transition from one register to another. At some point there is what is commonly known as a break, where the voice cracks or partially seizes up before moving into a new register.