Federal Tax Liens can really make your life miserable. When your taxes are not paid the IRS can establishe a lien against all of your assets (especially real estate). This gives the IRS the legal right to collect taxes from the sale of your assets, which includes just about everything you own.
The lien can be against you, your spouse, or your company. A lien against your company would seize your accounts receivable. 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 also do not want the extra work when the IRS comes in to take your money.