What happens if you don't have the money to pay the IRS for back taxes?

There are a lot of options. You can get on a payment plan, you can have them place you in "currently not collectible" status (meaning that they will acknowledge that you currently have a financial hardship and will leave you alone for a few years), you can try to make a settlement with them (an Offer in Compromise). It really depends on your financial situation. If you owe less than $25,000 you can call them up and get a five year payment plan wihout needing to provide any documentation to them -- it's guaranteed. If you want to do an offer in compromise or be placed in currently not collectible status you will need to provide a financial statement to them showing your current finances.