Why do some people pay in taxes and others receive a return?

I'm wanting to have a home business and having nothing but fits on trying to figure out all the income tax stuff. The IRS website isn't really helping me much and I've contacted several people and keep getting different things, as I know I will here but seriously. My mom has a home business and she always has to pay in at tax time. If I had a home business I know I don't have to file up to a certain amount but how do you keep track of all that and claim it? Would anything be returned or would I have to pay in. I'm wanting a home job but not sure about all the tax stuff. Please help and thanks mucho!!!!