Bias is "a particular tendency, trend, inclination, feeling, or opinion" about someone or something. When we discuss bias in media in the US, we are generally referring to conservative (also known as right) v. liberal (also known as left) bias, though there are many more ways to be biased and no one is truly free of bias.
Bias differs from fake news in that fake news is specifically untrue. Biased sources don't necessarily use lies, they just don't include the whole picture, only using the facts that support their viewpoint. By using only the facts that support their cause they are giving an incomplete and therefore inaccurate picture.