There are 0311's (infnatry) in both the corps and Marine Expeditionary Units. MEU's are not really "elite" in any sense. If you are assinged to a MEU, you will get a 6 month training course that will teach Marines how to work in a MEU. MEU's are excellent for a quick reaction force, therefore the training of a MEU is geared toward such.  For example, one course in the MEU training is on embassy evacuation, which is one of the primary tasks of a MEU. USMC Recon might fall under as the elite light infantry of the corps, but this is only due to the focus on small unit training. However, their job is just like their name says, reconaissence. Though I would imagine that light infantry jobs makes up most of what they are doing these days.

When you get assigned to MEU, do you stay there for a long time, does it become your permanent unit or you are in MEU just for a period of time and then sent back to your previous unit ? Can any soldier be assigned to MEU, or only experienced soldiers are invited, do they assign fresh recruits to MEUs ?

but the one thing that caught my attention is that Royal Marines and US Marines exchange officers pretty often, so there must be some similarities. Sure the USMC is a whole branch of the military, while RM doesn't have armor, nor air support, but basic infantry training is probably pretty similar, altough RM must be much more selective considering their small numbers.

There are some similarities between the Royal Marines and the USMC but they are not the same. The RM is more of a comando type organization ( a Damned good one too) and the USMC is more of an Infantry Unit with supporting assets. While the USMC is more than capable of conducting raids, The USMC is infantry first.

The RM is highly selective, much more selective than the USMC due in part to their primary mission and in part to their size.

 

Comparisons however should not be made head to head, The USMC is the best at what they do, likewise, the RM are the best at what they do.

What are the main infantry units of the USMC ? Are the MEU like elite units of the Marines, except for the Force Recon ? Are there any units in the USMC that are better trained and equiped than the others, not as a SOF unit, rather as an elite light infantry unit ? I'm very low educated on this subject, so don't flame me :D

In terms of quality, the US Army Rangers are on the same level as the Royal Marines. As are USMC recon units. But they do not have a similar tasking, so comparisons are difficult to say the least. But to answer your question fully, the USMC does not have a unit with a similar tasking of the Royal Marines.