Silly question, but what is the difference between permanent and semipermanent hair dyes?

[ThursdayNextIsMyHero]

Okay, I'm a hair-dying novice. I can't afford to go to a salon, so it'll have to be done at home. I have dark brown hair with quite a bit of noticeable grey. Looking at the dye available I couldn't work out from reading the boxes what the differences were between semi-permanent dyes (which the boxes said last up to 24 washes, which for me is about 7 weeks), and permanent ones which last for up to 8 weeks. Is one more damaging to hair? I had always assumed that the semi colours would wash out but the permanent ones would grow out. Is that right? But the timing on the boxes suggests they both last for a similar amount of time. I've bought a permanent one, and have done the allergy test, but now I'm wondering.

Any advice, s&b gurus?

[vodkaanddietirnbru]

This explains the difference