Similar to a levy - which involves intangible assets, such as your bank account - a seizure is the forcible taking of physical assets, like your home or car. Seizures usually happen in special or severe cases when someone continually ignores all of the IRS's requests to pay the outstanding taxes over a long period of time.
A seizure of your assets should not be taken lightly. After the IRS seizes your assets, they'll typically auction them off. They often get less than half your assets value, so they often seize everything you own including your home, cars, boats, jewelry, motorcycles, insurance polices, and even your retirement funds.