what is the dif. beween them
can anyone help me?

i wan to starrt this kind of things but i don't which one shall i use

I don't know how to said what actually different between booth of them. Booth of them are belong to the categories Source Distro, mean all the application inside that are compile spesific for U hardware so U can gain much performance compare to another distro like Redhat & Mandrake. For me if U " really2 " want to learn a lot of linux & are linux geek maybe U can try LFS but if U want to start much more faster & simple then that use Gentoo. For my experince using Gentoo for about 6 month it's a very good distro compare to RedHat & Mandrake & I have been learning a lot after using using it.

LFS your actually building everything yourself from scratch.. unlike Gentoo. LFS you have to have an existing distro installed. Read part of the LFS book at www.linuxfromscratch.com and you'll soon notice the difference between them.
Gentoo is more like a distro where you choose what you want with it.. unlike LFS where your choosing what you want, after you compile everything that is required by Linux to run...

Gentoo isn't completly as build-it-youself as LFS. But 90% of people are only interested in building the stuff after the base system rather than building the whole distro from scratch, so thats why Gentoo is cool. With Gentoo you boot into a base linux install (from the cd) and you just compile everything you want.