The correct answer has already been posted but I thought it might be helpful to explain it.
In any scale, each note can be numbered. For example, in the key of C:
C D E F G A B C
1 2 3 4 5 6 7 8
Where you start, C, is numbered "1." The second note in the scale, D, is numbered "2." And so forth.
The major seventh chord consists of key numbers 1, 3, 5, and 7. That's why it's called a "seventh" chord - the seventh note of the scale is played along with the root chord, C, E, and G. So, a CM7 (C Major Seventh) chord is C-E-G-B.
The DOMINANT seventh, however, flats the 7th note. Remember, the seventh note is B. Flatting the B note (which means dropping it 1/2 step) makes the 7th note Bb. So, a C Dominant Seventh is C-E-G-Bb.
If you are reading music and all you see is the notation "C7", what you need to play is the dominant seventh. (The flatted 7th) I think of the meaning of the word "dominant" means "stronger" - and it's the one you use when someone just throws the word "seventh" around without specifying. It's also the seventh you use when you're building a larger chord, like a 9th, 11th or 13th.
If you're interested:
9ths, 11ths and 13ths work the same way. You'd play the 9th note in the scale (obviously you'd have to start over, because you know there are only 8 notes to a scale).
C D E F G A B C D
1 2 3 4 5 6 7 8 9
The 9th note is D, as you can see - so add a D to your dominant seventh chord, and you have a 9th. (You always play 9ths, 11ths, and 13ths with the 7th.) Each level adds another note, which can start to sound a little muddled and complicated to play, but you don't have to play all those notes with one hand, and sometimes people either drop one of the notes that's supposed to be played to simplify it or double up on one of the notes to phatten up the chord.
