A Clash of Kings Questions (Spoilers)

I just finished reading CoK and the end with Theon and Reek confused me a bit. With so many names being thrown around casually the entire story, I sometimes lose track of who is doing what. Also, with the way the chapters are set up, it's difficult to find the part of the story where certain secondary characters are mentioned. So, to my questions; Is Reek the Bolton bastard that ran off with the lady from the North that had no heir and married her? I couldn't remember if Rodrik arrested him and brought him back to Winterfell. So, after Theon took control, he sent him off to find soldiers and he came back to take the castle. Were the soldiers just sellswords? It seemed like Reek's whole plan was explained in a jumbled dialogue paragraph. Why did they burn the castle? Can anybody clear this up for me?

The whole Brave Companions and Roose Bolton at Harrenhal was also a bit hazy, but I'll leave that for another time.