Bias is when subjective, anecdotal or otherwise non-objective factors interfere with our ability to make dispassionate logical and/or rational conclusions about reality in a way that often conforms with whatever schemas or heuristics we have already come to rely upon to survive more easily.
The above definitions of bias get at the heart of the issue better than I could ever articulate. Much of the focus appears to be on the failure to act/think rationally, which is absolutely true but I just want to draw attention to the fact that bias not only exists as a "negative" force, in that it causes people to think that some groups can't do something, but also a "positive" force in that it causes people with biases to give the benefit of the doubt to particular groups which removes opportunities for others
There is no such thing as totally "objective" factors. Every measured factor is a factor that we have made a normative decision to measure. Bias is a belief or reason for something that is drawn from the "wrong" normative values as opposed to the "right" normative values.
Bias is pervasive. Our mind isn't able to always (or even often) assess the exact truth about a particular fact, person, etc., and so we've evolved to take mental shortcuts. Not all biases are bad, and in fact many are good - e.g. we prepare to fight/flee if we see a tiger (even if they don't attack 95% of the time, it's critical to be prepared for the 5% that they do). As individuals, we have to (1) become comfortable with the fact that biases are an innate part of us if we are to talk about them honestly, and (2) realize that negative biases become much more harmful when they invade/pervade systems and institutions, such as our criminal justice process. Thus, everyone may have biases, but not all biases are equal in their effect.
I'll echo the definitions above, and emphasize how a) there are thousands of types of bias and b) it's impossible for anyone to be completely "unbiased." Therefore, diversity on a jury (ie, the widest range of competing biases possible) is critical...
I echo the above. The law and the processes through which it is enforced are also biased. Therefore, when biased jurors confront that systemic bias, they can either cause additional harm or act as a counterweight, depending on all parties involved.