Shimano Rear Derailleur cage sizes SS GS SGS

Shimano rear derailleur cage sizes (reference)

Short - SS

Medium - GS

Long - SGS

My (naive) understanding.

When buying a rear derailleur (Shimano code RD)
Also keep in mind the cage length
In general short cage (SS) will have a short looking cage and long a long lookingcage
My understanding is that longer cages can deal with bigger teeth gears (front and/or back)

Examples :
A short cage (SS) probably won't cope with a 34 teeth big gear on a rear cassette. Maybe best for 26t rear max (?)
A long cage will likely cope with a 34 teeth rear cassette

Keep in mind to take into account the size of your front gears
It's about how much chain the rear cage can deal with from both the front and rear gear sizes. 

Road bikes often seem to have a 'smaller' 1st rear gear (e.g. 26 teeth) and SS cages are often for them (probably the point is that SS will be quicker for changing gear)
MTB's often have a 32 or 34 teeth rear 1st gear (big looking) and will probably need a GS or SGS (can deal with bigger teeth on rear or front)

My current status on this subject : pretty unsure but I have a 34t rear cassette so going for SGS to be safe. Awaiting more wisdom.

Shimano FD-R443 front derailleur : a quick look

Shimano DR-R443 front derailleur for triple front rings
For road bikes with a flatbar
This one is compatible with 7-8 speeds. I think the newer version also fits 9 speed.
For double front rings you would choose the FD-R440

This is the front derailleur on my commuter bike. It seems to be specifically for road bikes with a flatbar. I guess intended to be used for a road-bike chainset with mountain bike shifters. Maybe the indexing is specifically set.

Good quality. I have seen some shops list it as a Tiagra component, but my research does not confirm this. Looks more like a component from no group.

Weight : mine is 105g

Shimano bike websites

Official Shimano Bike Websites
Shimano USA Shimano EU (english)

In my opinion comparing Shimano bike components is unnecessarily difficult including on the official Shimano sites. It's a pity for the general cyclist. Here are the official USA and EU (english) sites anyhow.

Shimano groupset list

My knowledge of subject : 3/10

I have compiled this list for my own use to try and make sense of the Shimano groupsets. The lists are from wikipedia (abbreviated). My understanding is that the groupset tend to mean the 'mechanical' parts of the bike. This would usually be the drivetrain and shifters and brake levers. (?)

Shimano Road groupsets (2013)	Shimano MTB groupsets (2011)
For 2013, Shimano road bicycle groupsets include: Dura-Ace  Ultegra [6800] (11 speed) Ultegra [6700] (10 speed) 105 [5800] (11 speed) 105 [5700] (10 speed) Tiagra [4600] (10 speed) Sora [3500] (9 speed) Claris [2400] (8 speed)	XTR (10) Saint (9) Deore XT (9/10) SLX (9/10) Zee (10) Hone (9) Deore LX (8/9) Deore (9/10) Alivio (8/9) Acera (8) Altus (8) Tourney (8)

Link to wikipedia page
RCUK article : buyer's guide to road bike groupsets