Major

The shape

A major chord is three notes — root, major third, perfect fifth. On the guitar it's usually played as a six-string shape with a few doubled notes.

How we score it

root     the note the chord is named after
third    major — four frets up from the root
fifth    perfect — seven frets up from the root
voicing  which strings you actually play

When to use it

Everywhere. The major triad is the default color of western harmony, and almost every progression in the library starts from it.