I have a small forward facing one, about the size of a cigarette packet.
It has an SD card in it, can't remember how big.
It can record over two hours. You can adjust the resolution to record for longer (not recommended, as even the highest res is low res).
It records in bites, I have set mine for 3 minute bites, when the card is full it will record over the oldest bite on the card. There are a couple of setting for the size of the bites.
I have had it for a couple of years, and paid about 25 quid. Newer ones have much better features.
A feature is does not have, that others do, is the ability to replay on the built in screen. With mine you have to load the SD card into a computer to play back. This is perhaps something to look out for.