The Elements of a Character

Good characters make good stories, and making well rounded characters that are believable does not happen by accident.  If the characters you create are vague to you, then they will be vague to the reader.  Maybe the opposite case is more typical: you over describe your characters to the point to where your reader has a photographic mental image.  It is better to show your character.  For example, if your character is vain, then show it your characters actions and let your readers "discover" it.

I use the acronym PHAM to recall the elements of a character:
P - Past
H - Habits
A - Appearance
M - Motivation.

Past:
All characters have a past and it makes them who they are.  If a character is spoiled, then there must be past events that made them that way.

Habits:
Does a character like to bite their nails, twirl their hair, stay up late, etc.  Everyone has different habits, and seeing them make your characters more believable.

Appearance:
This is simply how your character looks: face and body shape, age, hair color, etc.  I would also include how the character dresses.

Motivation:
This is the key to your character.  Why is your character doing their actions?  For example, not everyone attends college for the same reason.  Some are their because they want to get a better job, some to better themselves and increase knowledge, and some because their parents told them that if they don't then they need to move out.

Tips:
1. give your "hero" at least one bad trait, no one is perfect.
2. Give your "villain" (at least) one good trait, no one (hopefully) is purely evil.