A couple of those subdirectories there contain a README file which you should read, but generally speaking:
vanilla - implies a plain, unpatched, unmodified kernel, same as from kernel.org
tuxonice - is a set of patches used to improve (or cause to work properly) sleep/hibernation functions, usually on laptops.
hardened - usually implies special security features or patches to make the kernel more 'bullet-proof' against attack or exploits. Maybe SELinux (SEcure Linux) is involved too, which provides extra access control security features.
zen - I have no idea, but this one has a README file.
gentoo - I guess this is whatever the default Gentoo kernel is, when it is not one of the above.
Someone who is more familiar with Gentoo may be able to give you more details about how/what exactly these particular kernels do that makes each of them more or less desirable for a given system.
Last edited by GrapefruiTgirl; 08-13-2010 at 06:05 PM.