We'll start with the basics.
He teaches economics at Randolph-Macon College in Ashland, Va. Brat, who has a PhD and a Masters of Divinity, teaches mostly introductory economics classes at the college, a small liberal arts school outside Richmond. His faculty Web site (which hasn't been updated recently), features photos of Adam Smith, John Calvin, Friedrich Hayek, and John Maynard Keynes in the four corners. (If you're curious, his students give him moderately decent ratings as a professor.)
Brat was backed by the tea party. In addition to being backed by prominent conservatives (Ann Coulter, Laura Ingraham), Brat was backed by grass-roots tea party groups as well. As the National Journal notes, Cantor's district became more conservative in the most recent redistricting, including adding new areas to his district, making an ideologically-driven challenge more possible.
In May, Cantor addressed the state Republican convention, where he received a negative response from some of the activists in the audience.
Brat is not a "liberal college professor." No doubt hoping that conservative voters would bail on Brat's candidacy, Cantor labeled him a "liberal college professor," which FactCheck determined was inaccurate. The site asked conservative economist Richard Rahn for his assessment of Brat and was told that "he always portrayed himself to me as very conservative."
Brat was massively outraised. According to his most recent filing with the FEC, Brat raised only about $206,000 through the middle of May. Cantor, on the other hand, raised $5.4 million this cycle.