"What are these? They're not really frogs, are they?"
-- Harry Potter (PS6)
Chocolate frogs are a very popular sweet made from chocolate in the form of a frog. They come with a collectible card of a famous witch or wizard in each pack (PS6, GF23). Chocolate frog cards are sold on the cart aboard the Hogwarts Express as well as at Honeydukes in Hogsmeade.
When Harry first met the chocolate frog, he asked Ron whether or not it was a real frog. It's not (PS6). The frog became one of Harry's favorite sweets (GF23).
Also, a chocolate frog was useful in fighting off his dementor-boggart when Lupin taught Harry the Patronus Charm (PA12).
Commentary
Notes
Chocolate frogs are J.K. Rowling's favorite candy as well as Harry's, but only because she would like to collect the cards (BN, Sch2)
A reader let us know that chocolate frogs -- decidedly Muggle ones -- are a type of sweet available in Australia. Greg writes:
As for Chocolate Frogs in "The Journey from Platform Nine and Three Quarters", these did not strike me as strange or interesting at all. I don't know about the UK but in Australia there is a confectionery known as a Freddo Frog (see https://en.wikipedia.org/wiki/Freddo). Apparently it has been introduced to other markets, although I don't know about the UK.
The idea for Chocolate Frog cards is supposedly a tribute on JKR's part to Monty Python:
"British comedy is an obsession of mine. I love Monty Python." (Sch2)
The effects of chocolate on the brain are well-known and documented.
Chocolate frogs -- Wizarding world ones -- are available for sale in the US and UK with famous wizard cards in them.