Inversions are playing a chord, only your root ain't on the bottom.
For example, if you're playing an A Major chord, but you hit the low E string as well, You're playing A maj 2nd inversion. Because the E is the fifth of an A chord.
If your lowest note is the third, it's called 1st inversion, and is notated by a superscript 4, because the interval created is a 4th. C#, E (min 3), A (4th).
If your lowest note is the fifth, it's called 2nd inversion, and is notated by a superscript 4/6. because you create a 4th and a 6th.
E, A(4th), C# (Maj 6th from E)
All in all though, it's inverted merely on the basis of the bass note, everything else can be in any order, and it's still named the same way.
It gets more complicated with 7ths, but I'm sure you'll get it.