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 just one short step away from 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. The banks don't want the extra work when the IRS comes in to take your money.
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.