This was answered pretty well somewhere else, but I can't remember where, so I'll give you what I remember/what I've learned in lectures at CTC...
Each Cdo unit has a Recce Troop, generally made up of 6 four man teams, each team led by a Sgt or Cpl who are normally MLs, although sometimes other specs. The rest of the men are Marines from various specialisations, mainly GD or PW (Snipers are a specialisation within the Platoon Weapon specialisation - it's bollocks that you have to be a sniper to get in, though snipers may have an advantage in that they're already further trained in many of the techniques required for recce duties eg. moving undetected, OPs etc), though I'd guess there are also others. You can apply for Recce Troops, and go through a selection similar to applying for specialisations such as ML.
Once you're in, your training will be geared towards, oddly, recconnaissance and information gathering. There's a lot of responsibility on a lowly Marine in that you're expected to find suitable start lines and FUPs for the main forces, advise the operational commander on the ground and enemy forces, lead forces into position, etc etc. You'll be operating ahead of the main force, cut off to a certain extent, and doing an important job, so you generally get to use quite a lot of gucci kit (but you also have to carry it

). Most are also para trained.
BRF is an extension of the Recce Troops, basically doing the same job but at Bde level rather than Unit. Afraid I don't know much about this one, a couple of my Cpls have served in Recce Troops (both snipers) but not BRF. Recce Troop is the first step in that direction though.