Only for domestic flights. The U.S. Department of State says the wallet-sized passport card is not valid for international travel by air; it is meant for U.S. citizens entering the United States by land or sea from Canada, Mexico, Bermuda, and some Caribbean destinations, according to the State Department. For flights inside the United States, the card is accepted as an alternative to a REAL ID.
So a passport card can get you through TSA on a domestic flight and back into the United States by land or sea from those nearby destinations, but flying to another country, including Canada or Mexico, requires a passport book. The card costs less ($65 for a first-time adult card versus $165 for a book) and has the same validity: 10 years for adults and 5 years for children under 16.
Source: U.S. Department of State, “Compare a Passport Card and Book”
Image: Specimen U.S. passport card, U.S. Department of State, public domain, via Wikimedia Commons.