There are several things to consider when purchasing an outdoor rated camera:
Ingress Protection (IP) Rated: At minimum you want a camera that is rated IP65 ("dust tight" and protected against water projected from a nozzle), however, most cameras I have seen are rated one up at IP66 ("dust tight" and protected against heavy seas or powerful jets of water).
Megapixel: Most HD cameras will be rated by megapixel and at a minimum you want either 1.3 or 1 megapixel. I've seen cameras go high as 8 megapixel, but they are also more expensive. Older, analog cameras will usually be rated by TVL (TV lines) and those do not produce images that allows for easy identification.
Infrared (IR): You will want to see in the dark, IR cameras will allow you to do this, but usually at a lesser resolution or quality of image. Images in IR are black and white, although, there are new technology coming out that will colorize night time images. Dependent on the field of coverage (backyard length, driveway length, and etc...) you will have to research how far a camera can see with IR enabled.
These are secondary factors, since you are looking for a camera that can connect via WLAN.
Ethernet/Network-based: Most of the time this allows for an easier install (no need for additional hardware, plug & play), you can configure the camera via the web browser and view the camera remotely.
Power-over Ethernet: If your camera is Ethernet-based--you will only need to run Cat5e/6 to your location. Data and power will run on a single Ethernet cable, no need to run power or have an outlet at your camera location. However, note that if your camera doesn't include a POE injector then you will need to get an injector or a POE-enabled switch.