When your taxes are not paid, a lien is established against all of your assets, giving the IRS the legal right to collect taxes from the sale of your assets, which can include a majority of what you own, especially real estate.
The lien can be against you, your spouse, or your company. A lien against your company would seize your accounts receivables. At this point everything you own is at risk of becoming the property of the United States government.
Liens filed against you by the IRS also show up on your credit report and often prevent you from opening a checking account or borrowing against any assets, like your home.
With a Federal Tax lien on your record you face considerable financial challenges such as the inability to get a reasonable loan to purchase a car with high interest rates, not being able to buy or sell any real estate, etc.