Skills are central to character customisation in Skyrim. They are split into three categories: The Mage, The Warrior and The Thief. Each category has six skills, for a total of 18 in the game. Each skill begins at level 15 (this level is boosted depending on your race) and is capped at level 100.
You upgrade a skill by using it (for example, attacking enemies with a one-handed sword improves one-handed skill), reading specific books, and purchasing training from experts in and masters of that skill.
Your skill level affects your aptitude, and as you reach certain skill level boundaries, you unlock perks which further customise your character (for example, the one-handed skill has a perk increasing damage dealt with one-handed weapons).