A single-axis tracker attempts to optimize solar exposure by rotating about the vertical (a.k.a. Azimuth), or north-south (a.k.a Polar) axis. The first picture shows an azimuth tracker. It has a fixed angle slope, which is optimally set to match the latitude, and it rotates on a single pole type mount.
The second picture shows a polar tracker.
Here the axis is tilted to match the latitude, and oriented to point to the celestial north or south pole (depending upon your hemisphere).
The next picture shows a horizontal single axis solar tracker.
At the equator this would be the same thing as a polar tracker, but for non-tropical locations we'll call it a horizontal tracker, knowing it to be a special type of polar tracker whose pitch is equal to the lattitude.
The closer you are to the equator, the better it is to use a polar axis, but Azimuth mounts are sometimes the best to install in a given location. For a specific application like a solar oven, an azimuth tracker works better because the oven doesn't have to tilt. Roof mounted systems will generally use polar tracking, while ground based systems may use either approach. Azimuth trackers are limited in the size of the array they can handle because the array is structurally supported at only one point.