Minor chords are comprised of at least 3 notes, the Root, which gives the chord it's name, a Flatted 3rd, and the 5th. For instance a Am ( A minor) chord would look like this:
--0--
--1--
--2--
--2--
--0--
--X--
the key of A contains the following notes:
A B C# D E F# G# A
There is an A on open 5th string, an E 2nd fret, 4th string, an A 2nd fret 3rd string, a C 1nd fret 2nd string, and an E open 1st string. A is the root, C is the flatted 3rd, and E is the 5th. The only diference between an A and Am chord is the C# is flatted, which makes it a C
Now lets look at an Em (E minor)
--0--
--0--
--0--
--2--
--2--
--0--
The key of E contains the following notes:
E F# G# A B C# D# E
an Em contains 3 E's open 1st and 6th strings, and 2nd fret 4th string, a B 2nd fret 5th string and open 2nd string, and G open 3rd string. E is the root, G is the flat 3rd, and B is the 5th.