No, this isn't going to be a bunch of technical details about the M-16, as those are easy enough to find elsewhere. However, if you're still interested, here is...

A little background info:

The M-16 was first used by US armed forces in the mid-1960s, and has since become the light infantry weapon of choice (though not necessarily the weapon of issue - most european nations use other (european made) rifles) for most western armies. The different versions of the M-16 weight approximately 3 kilograms (a little under 7 lbs). The standard M-16 is 99 centimeters long (about 38 inches). A standard ammo clip holds 30 rounds of 5.56 ammunition, with smaller clips of 20 and 6 rounds also available. The M-16 has a Safe mode, a Semi-Automatic mode, and an Automatic mode.

Why different versions?

There are three main variants of the M-16. The Assault Rifle version, while long and somewhat bulky, is extremely accurate at a range of up to 100 meters. The Light Assault Rifle, or LAR, version has a buttstock with 4 positions, which allows for an easier-to-carry weapon but, when opened to the relevant position, allows for effective aiming and firing. Last is the heavier version of the M-16, which is used primarily as a sniper weapon. This version, much like the standard M-16, lacks the collapsable sliding buttstock, and is therefor bulky.

Some slightly more practical information

This researcher first encountered the M-16 in IDF (Israeli Defense Force)basic training. As the Israeli infantry divisions all use the M-16, the more convinient LARs are allocated to actual combat units, so, in basic training, troops use the standard version of the M-16. It has been observed, on many occasions, that the standard M-16 is also known as the "Sorry" weapon-walking in a crowded environment, i.e. a bus, sounds much like this-"Sorry, pardon, sorry, whoops, didn't mean to hit you with that sir". However, despite being bulky, the standard M-16 is very accurate, easy to fire, and easy to care for.

While the M-16 requires constant cleaning in a combat environment, one can be safely stored for several months and not require cleaning before it is used. This makes the M-16 effective as a standby weapon, as well as lightening the load on troops (this is especially notable in basic training- if you hadn't been in the firing range that day, you could get away with not cleaning your weapon that day.

Probably the most convinient version of the M-16 is the LAR, or the light assault rifle version of the rifle. While not as accurate (due to the longer distance between its sights, the standard, longer M-16 is more accurate), its folding buttstock is an incredible advantage. The LAR is sometimes adapated to use a barrel with a 7:1 ratio of aspersions rather than the standard 12:1- This allows it to fire bullets with greater velocity, and allows it to use "Type-B" ammo (That being ammunition with more gunpowder).

And, in conclusion...

Overall, while it is slowly being replaced as the preffered for light infantry by newer rifles (*), the M-16 is still an effective weapon, and one worthy of a place in military history.

In two words, the M-16 is perhaps best described with the classic phrase, "Mostly Harmful" (Douglas Adams quip. Apologies in advance.)