Each chord---A, B, C, D, E, F, G---can either be major or minor. The major chord is, simply, the chord itself. Sometimes it is notated as "Amaj," but oftentimes it will be listed simply as "A." Major chords have a lighter or fuller sound than minor chords, which often are a bit darker, or more sullen sounding.
Because there is a whole step between A, C, D, F and G, these are the chords that have sharps. A sharp chord is a chord played one half-step above the major chord. A half-step means that the frets are next to one another; a whole-step means that you skip a fret. For instance, a whole step is from the first to the third fret, while a half-step is between the first and the second fret.
There is only a half-step between B and C and E and F, which is why there is no B sharp or E sharp. A, B, D, E and G have flats, which is a half-step below the major chord. Again, since there is a half-step from C to B and F to E, there is no C flat or F flat.
A sharp and B flat are the same chord.
The seventh and extended chords (fifths, ninths, elevenths, thirteenths) add another dimension to standard guitar chords, and are often used by many jazz, blues and progressive rock musicians. These augmentations add subtle yet quite noticeable differences to chords, and are usually the last of the guitar chords a player masters.