To make your website text as readable as possible it's important to use the right number of characters to display on each line.
If your lines are really long, the reader will have a hard time finding the next line to read (because it's too far away from the beginning). If they're too short however, the reader will lose their rhythm because their eyes will have to track back too often.
But is there a magic number to aim for?
Most experts seem to indicate that having anywhere from 45 to 120 characters per line is fine. In other words, it won't cause the reader any noticeable problems if you keep it within that range. 55 to 65 characters is the most optimal for print, but there are studies that seem to suggest you can get away with a slightly higher number of characters on the web. The official W3C recommendation is to keep line length under 80 characters.
Given all that, I tend to aim for around 70 characters per line.
I do make an exception when targeting phones though. Rather than making the font really small to try and accommodate the 70 characters per line rule, I'm ok with ending up at around 50 characters per line instead.