The idea is that having small lines attached to the end of letters, will have a big influence on how readable that font is.
Most ads today are written with sans serif fonts; in the past, serif was more popular. Generally it's believed that serif fonts are easier to read in print, while sans serif fronts are easier to read online.
Here's a summary of the differences:
Serif vs. Sans: the final battle | Webdesigner Depot
Now generally I agree with what this infograph is saying. Sans serif is easier to read online because it looks cleaner at low resolution.
But here's something else to consider:
What if computer screens eventually DO get to the point where they have the same resolution as print?
Should we all switch to using Serif then?
What got me thinking about this was the difference in reading the font Georgia on my PC vs. on my retina MacBook Pro. On my mac, it's one of the most beautiful fonts I've ever seen. On my PC, which has a low res screen, it looks absolutely awful. Originally I thought it was because Microsoft used a different 'version' of the font, but I later figured out that it was just because of the different screen resolution.
What do warriors think about this?