I have a WheelWitness that I got on Amazon. It records in HD to a micro SD card. The recording is broken up into continuous segments, the length of the segments is in minutes and is user selected. When the card fills up, it replaces the oldest segment. When you plug the card into your computer, each recording segment appears in a folder for easy browsing, like searching for movies on a smart phone. Many of these cams are similar in features and have an option to also run while the car is off or begin recording when the car is bumped. They can also record speed and gps if you want.
Having had this for a while, in general these dash cams are good for reading license plates while the other car is stopped or moving at a similar speed. Or, for being a witness to an accident. Not so good for reading plates on a car speeding by, so you may only get a car description and partial plate on a hit and run if that happens. Night vision on some brands is bad. There are video reviews of different brands on Amazon with actual footage of driving and incidents.
The technology is new and improving. When they get closer to 4k resolution someday, the detail on moving vehicles will be better. I make a 4k road trip every summer from TX to PA, and feel more confident knowing the cam is recording what is happening around me. And since most accidents occur near one's home, they are good insurance around town for the price.