The D chord on the guitar has several different variations that a good guitarist should be aware of. I'll go through them all here in as much detail as necessary, while making sure that you get to the fretboard quickly.
The first thing to keep in mind is that in a standard tuning the third string played open (without a fret pressed down) is your "open D". Keep that in mind going forward.
The diagram show's you all the notes for the D string on the guitar. This isn't something you need to know off the top of your head right now, but memorize it when you get a chance.
The D ChordThe term "D Chord" simply means that you're playing a chord in the key of D, having a D as the root note.
This means that there are a number of D chords, some of which are more typical than others. A more modern pop/rock guitarist will see certain variations of the D chord more than others, so those are the ones I want to focus on.
1. Simplified Major ChordThis is the most typical form of a D that you will play. Make sure you mute the 6th string, as it's omission actually makes for a cleaner sounding chord. The top two strings are muted as well.
2. Tenth Fret Form Power ChordIt's a barre chord, but it's more useful than many of the other D chord variations you'll find. Very simple and also typically utilized, especially in the realm of hard rock. Mute the four high strings.
3. MinorYou might be familiar with this already, but if not, no worries. It's just one extra note added to the simplified D Major.
4. Open Drop DThis might be a little strange, but if you want to get into rock and roll, you've got to get cozy with the drop D tuning and be able to switch back and forth between drop D and a standard tuning pretty quickly. For a tutorial on the tuning itself (if you're not already familiar with it), you can check out this quick write-up on the drop-D tuning.
The chord itself is wonderfully simple and you have two different options. I'll just call these options, "full" and "heavy".
The full version of the open drop D chord is the following diagram. The top five strings are all incorporated, basically adding the low A and low D to a simplified D major chord: