what are those little things?

[e-rider]

in the end of shimano cable ferrules - one is white (and hard) and the other is black (and soft) - both very small with a hole in the middle for the cable to pass through?

do I really need them?

and another question:

lets just say I'm a bit incompetent and I can't cut outer cable to the perfect length - is it better to go long or short?

 

[gavintc]

in the end of shimano cable ferrules - one is white (and hard) and the other is black (and soft) - both very small with a hole in the middle for the cable to pass through?

do I really need them?

and another question:

lets just say I'm a bit incompetent and I can't cut outer cable to the perfect length - is it better to go long or short?

I think you mean these.

Stops the cable end fraying:

http://www.wiggle.co.uk/transfil-pack-of-10-anti-fray-inner-cable-end-caps/

Longer is generally better than too short, but too long and the cable could make an unnecessary tight curve.

 

[zexel]

Do you mean the rubber grommet type things?

... to stop the cable touching the frame?

 

[e-rider]

no, I don't mean cables ends or rubber gromets (donuts), I'm not that stupid! Look inside the black outer cable end ferrule and you'll find (on Shimano ones) several little things hiding inside!

 

[Zoiders]

Tiny little wiper seals/grommets to stop water ingress into the outer is my guess, they could even be PTFE impregnated to reduce friction.