There are three main character attributes in Skyrim:
Health, represented by a red bar
Magicka, represented by a blue bar
Stamina, represented by a green bar
Attribute bars appear at the center bottom of the screen when they change : Health appears when you are hit, Magicka when you use spells and stamina running or fighting.
Health[]
Health is the most important attribute. Regulates the damage you can withstand before dying and then losing. To survive longer is better to protect themselves with suits of armor or increase the health. Health regenerates slowly over time, and only out of combat.
Stamina[]
Stamina is necessary to running and fighting, but also to sprint, jump and block shots. Without stamina, you can not do these things and you have to wait for it to regenerate, but it happens quickly, even in battle.
Magicka[]
Magicka is necessary to cast spells. It regenerates in and outside of combat, and not as fast as Stamina. Is not necessary to use the dragons shout, as they are completely independent of the types of magic spells.
Note[]
If you are a vampire, you can not regenerate health, stamina, or magicka in direct sunlight.