THe short version is that if you get income from something, you are supposed to declare it as *something*. If you make necklaces out of soda can tabs and sell them at the flea market and make $15, you are technically supposed to tell the IRS about it.
It's pretty unlikely that anyone will come after you for not telling the IRS about chump change, particularly if you do it in cash and don't have a paper trail anyway. But if you have deposits showing up in your bank account that are bigger than your buddy reimbursing you for his share of dinner, it's a good idea to declare them. It doesn't mean you'll actually owe any more in taxes necessarily - if you brought in $1000 by selling babies but spent $1000 on new snakes, it's a wash. But you should still declare it.