Liberia has two units of currency; the Liberty dollar (L$, or LD) and the US dollar, which is accepted in most places. There are between 60 and 73 Liberty dollars to one US dollar. For small purchases such as newspapers, shared taxi rides and cheap meals, Liberians use L$. For anything else, it's US dollars.
If you have a Visa (not MasterCard) card, Ecobank's ATMs are reasonably reliable in Monrovia. Elsewhere, carry cash. Western Union and Moneygram operate in most towns.