Just so...here's a little more from www.tabcheck.com:

The best description of a ghost note, is a note that is felt but not heard. You will play the note softer, and without emphasis. The note is usually in-between 2 parentheses. In addition, notes in parentheses could mean optional notes. For instance, if a particular riff is repeated, but sometimes the guitar player throws in some additional notes, those additional notes may be in parentheses. Do keep in mind the use of parentheses for bent notes as well.

The example below could mean either a ghost note or an optional note. There is no way of telling without a recording. And really in this context, do either and it will sound fine.

E--------------------
B--------------3-----
G--------------2-----
D--------------0-----
A---3--(0)--3--------
E--------------------