A good chart describing video resolution, found on Wikipedia. Note that 1080 and 720, as commonly-used names of resolutions, describe the number of horizontal lines, while 2k and 4k describe vertical lines.
720: 1280 x 720
1080: 1920 x 1080
2K: 2048 x 1080
4K: 4096 x 2160
This is a good chart, too, packed with info, but it can make you a bit cross-eyed.
A note on 720 x 480 versus 640 x 480
In situations where video has been shot in 720 x 480 and plays back at 640 x 480, it’s because the recorded pixels are rectangular and the playback pixels are square (and slightly larger). The playback program does the conversion automatically. More here: http://forum.videohelp.com/threads/312656-Can-someone-EXPLAIN-the-whole-720×480-thing-to-me.