What kind? Do you like them? Been trying to convince the wife we need them in all 4 vehicles and on my helmet
I've found that the mid-price-point ones work just fine (Rexing, Viofo). These are basic cameras that use a micro-SD card to record what happens in the background, and if you need to access the footage, simply pull the card, insert it into a reader and plug it into your computer. There are generally options to record the sound or not, clip length, and file overwrite - also known as loop recording - is usually standard. File overwrite simply means that once your 16 GB SD card is full, the camera loops around and starts recording over the top of the oldest files first so you can leave the card in there indefinitely.
There are versions that offer "Wi-Fi" access, but you have to join the camera's proprietary Wi-Fi network which is usually slow AF, then scan the files to find the one or two that you want to save. To me, it's simpler and faster to do that on my iMac than on my phone.
The basic cameras really provide front windshield footage. The next step up in terms of dash cam functionality also has a rear-facing camera to place on the back window, but this involves running a wire along the underside of the running boards to complete the circuit. Not terribly complex work, but a lot of folks don't have the tools or skills to do that kind of mechanical install, and if you're putting it in an SUV you've got to allow for the hatch to open and close, etc.
Some folks get by with an "Uber-style" camera that records what's happening outside the windshield on the front-facing camera as it's recording what's happening in the interior of the vehicle (and out the back window) on the rear-facing camera. Lots of choices available to you if you decide this approach makes sense for you.
I also generally order a "hard wire" kit that runs a dedicated power line down to the fuse box of the vehicle vs. going with the "cigarette lighter" power cords that always come standard with the dash cam kits. I hate loose wires in the interior, and frankly finding a vehicle with that kind of power port is getting harder and harder to do. Those hard wire kits are generally less than $20 and with a "spudger" you can run one along the inside of the windshield next to the headliner in your vehicle pretty easily. Just push it into an "active" location on fuse box on whatever side your footwell fusebox is and you should be good to go. Your vehicle's owner's manual will tell you the location of the active fuses and what they're being used for, and I usually add it to a non-critical zone so if the fuse blows I'm not taking out my headlights, etc.
Free advice: if you're ever in an accident, sometimes the cops will see the dash cam and want to take it as evidence. Simply tell them that there are personal items recorded on the device and that your attorney will be happy to provide the relevant footage to them in support of the investigation. That's important unless you want every conversation that took place in the "privacy" of your vehicle turned over to the police.