I've seen a lot of members dicscussing the rel="nofollow" and rel="dofollow" link relations in regards to linking back to their own site at the bottom of their clients website, however, I'm unclear on what these actually do.

Could someone kindly educate me on what these link relations are for, what they do [both negatives and positives] and when they should and shouldn't be used, please?

Thanks very much.

Andrew Cooper