Joined: 13 Jun 06
29 Nov 09 9:00 pm
The two ratings are very similar - they are a combination of factors (such as how well optimized the top ten results are for that keyword etc.) that give you a rough idea of how difficult it might be to rank highly for that keyword phrase.
Of course just because a phrase might be competitive or difficult doesn't mean you can't give it a whirl - just look for related keywords using the keyword finder in Traffic Travis and find some high volume phrases in the same niche.