In legal terms, there's no such thing as a "temporary green card." However, it comes up as a slang expression for what's known as "conditional residence" in the United States. This article will explain what conditional residence is, and why "temporary green card" is a misleading term for it.
In most cases, when someone obtains the right to a U.S. green card; perhaps through a family member, employer, or based on having spent a year in the U.S. as an approved asylee or refugee; that green card represents "lawful permanent residence." In other words, the recipient is allowed to stay in the U.S. for life, assuming the person complies with U.S. law and doesn't become deportable or move to another country and abandon U.S. residence.
However, for two categories of people, U.S. law places conditions upon what would otherwise be U.S. permanent residence, including:
The "condition" placed on both these groups is that, at the end of two years in the U.S., they must submit an application to USCIS to recognize that they continue to be eligible for U.S. residence. Once that application has been approved, their residence becomes permanent.
During the two years of conditional residence, the person has all the rights and obligations of a regular green card holder. They can carry a card that looks much like a normal green card and work in the U.S. without special permission.
What's more, they can count their years toward the five normally required for U.S. citizenship (or three years in some cases, based on being married to and living with a U.S. citizen all that time). Once they get U.S. citizenship, looking back, it will be clear that nothing was "temporary" about their U.S. residence.
The only "temporary" aspect of conditional residence is that conditional residents have a two-year expiration date on their green card. And if they were to sit back and do nothing, it's true that their green card would be "temporary," in the sense that they would lose status in the U.S. and become removable.
But in most cases, the conditional residents can, should, and will take the next steps in the application process; in other words, file Form I-751 with USCIS if the green card was based on family, and file Form I-829 if the green card was based on investment. Thus their status in the U.S. is only temporary if they make it so; or if, during those two years, they cease to be eligible for the status they were conditionally approved for.