When your taxes are not paid the IRS can file a lien against all of your assets. This gives the IRS the legal right to collect taxes from the sale of your assets, which includes just about everything you own.
Liens filed against you by the IRS show up on your credit report and often prevent you from opening a checking account or borrowing against any assets, like your home. The banks don't want to loan you money when the IRS is in first position.
With a Federal Tax lien on your record you can't get a reasonable loan to purchase a car. Think about paying 18-22% interest on a car that is already too expensive. You definitely cannot buy or sell any real estate. The list is endless.