(Responses from: Mark Zenier (firstname.lastname@example.org or email@example.com))
The first difference is that a lot of the world runs on 50 Hz power as opposed to North America's 60 Hz power. In the olden days, before active power supply regulators got cheap, it was decided that the vertical scan rate match the power supply frequency, so that ripple in the power supply wouldn't produce obnoxious visual effects.
So the PAL/SECAM signals have 50 vertical scans per second.
I don't know the exact reasoning, but the horizontal scan rate is close to the same. 15750 (now 15734) for 60 Hz, and 15625 for 50 Hz systems. My guess is the tradeoff between cost (50 years or so ago) and audibility for a large portion of the population.
So 50 Hz systems have more lines - 625 vs. 525 lines for 60 Hz systems.
The second difference is that European TV channels are wider. 7 or 8 MHz compared to the North American 6 MHz. Video bandwidth is limited to 4.2 MHz in a 6 MHz channel, but can be as much as 6 MHz in some of the 50 Hz systems. (Note: Systems is plural. There are many different European systems with incompatible color and sound transmission methods.)
As for the quality, if you move a little farther away, so that a pixel on each system subtends the same angle, NTSC doesn't have a poorer picture, just a smaller one.