It's an update on the Google algorithm. Exactly what is updated, many doesn't know. But it seems like most back-links from forum profiles and blog comments, now doesn't deliver the amount of juice it did before. Have a search on it on Google for more info.
Google panda is the updated algorithm of Google which works as a filter to detect copied content and to rank your site according to your site quality in the search engine.
this one is been brought up many times in this forum.. i suggest to use the search box or browse the forum..
Google Panda is latest Google algorithm... It is a fight against Spammy / Junk sites. This algorithm is promoting quality websites and pushing down poor quality websites. You can get more information about Panda update with a quick Google search.