What is a "billionaire's tax," and how would it work?

Democrats are considering a new tax on billionaires that would help fund their proposed $3.2 trillion social spending package. Although some critics of the plan say it opens the door to tax hikes on a broader swath of Americans, the proposal would only impact taxpayers with more than $1 billion in assets, or those with incomes of more than $100 million for three consecutive years. 

In all, the tax about apply to roughly 700 of the wealthiest people in the U.S.

A key change under the plan is that tradable assets, such as stocks, would be valued annually. Billionaires would be taxed on their gains whether they have sold the asset or not. Under current law, a gain is only taxed if it is "realized"when its owner sells the asset and books the profit. Unrealized gains — stocks or other investments that rise in value and that the investor holds onto — aren't currently taxed.