an explanation of a DM

First of all, I know that DM means Designated Marksmen.
Second, what I want out of this thread is a basic explanation of a Designated Marksmen, my friend says it is someone who
stays in a certain area and protects it. However, my friend is renowned for giving unintellectual insights.
also something I would like is types of the guns DM's use, maybe some basic tactics that they might use.

Okay, a designated marksmen, or DM, is similar to the role of a sniper but instead typically moves with his squad, instead of being mostly stationary. So, in essence, your friend has been mis-informed. The DM can play the role of sniper, but he is more limited as to what he can do as opposed to a player who came dressed to be a sniper.

The DM typically uses a DMR, or Designated Marksmen's Rifle. These are typically long-barreled AEGs, like and SR12. Here are some examples:

http://www.airsoftgi.com/product_info.php?products_id=2309

http://www.airsoftgi.com/product_info.php?products_id=2355

http://www.airsoftgi.com/product_info.php?products_id=6097

http://www.airsoftgi.com/product_info.php?cPath=139_24_211&products_id=3601

I feel like I'm not explaining it fully, so if somebody can help,
"Throw me friggin' bone here"

In my knowledge, a Designated Marksman fills the role of sniper or long-range marksman in a squad format, when a true sniper would not be present. It's usually to give the squad some long-range capability, and it helps with balancing out the squad or fireteam. So, yes, your friend was wrong.

Guns, guns, guns. As M14 said, any kind of long-barreled rifle will work. M16, M14, G3, M14 EBR, SR-25, etc.

Here's (from Wikipedia) what they say:

"The DM's role is to supply rapid accurate fire on enemy targets at ranges up to 800 metres (870 yd) with a highly-accurate semi-automatic rifle equipped with a telescopic sight. Like snipers, DM's are trained in quick and precise shooting, but unlike the more specialized "true" sniper, they are also intended to lay down accurate rapid fire."

Hope this helps, and feel free to correct me on anything (Jarhead we need you!).

Your friend deserves a severe smacking for such insolence! Just kidding, but he is wrong. These guys are spot on; a DM is Designated Marksman in a (usually) a squad-sized element. Generally, DMs are the guy with the best marksmanship in the squad and usually have longer-barreled rifle replicas.

will post more later