If I put a link on my site to your site, but you don't link back to mine, that is a one way link. If you do link back to mine, it's a reciprocal link. When they see reciprocal links, the search engines figure the sites are just doing it to get links, so they discount the value.
I don't understand the logic behind this tho.... If I link to you and you link to me it shows community. People buy and sell one way links for big money. I see them advertised. Should knowing one way are sold the most, they should not have as much value.
one way link means that you find another site that will post a link to your website without being necessary to be a reciprocal one and it's obviously that one way links are more important than reciprocal ones ... by far.
forum signatures are a long story and we'll better discuss about them in another thread.
I tell you a little secret about them... it is the admins' option to choose weather the signatures will be seen by search engines bots or not...
Oneway link means that you gain a link from other but you don't give link back it site. In this condition your site get Link Juice from backlink site but your site not transfering Link juice. So your PR will be increase.