A font is a specific size and style of type within a type family [WordNet-Online]. There are tens of thousands of font families and perhaps hundreds of thousands of individual fonts (nobody really knows for sure). Your operating system came with several fonts pre-installed. Keep reading to learn more about getting and installing more fonts and fonts in general.
SERIF & SAN-SERIF
Serifs are the small projected features of type; the lines at the base of the letters and at the ends of the shape that forms the letters. Serif fonts are commonly used on printed material with lots of type; such as a newspaper, or a novel. San-serif fonts are ones without projected lines. San-serif type is common on web pages (like this one) and on printed material like billboards, posters and newspaper headlines. An easy way to remember the difference between the two is to think of serif type as the ones "with feet," and san-serif as the ones "without feet."
There are a myriad of font formats including FreeType, OpenType (OTF), the Web Open Font Format (WOFF) and many more [see Wikipedia]. The "TrueType" font format (TTF) was the defacto-standard, but now has now been largely replaced by OpenType (OTF). So, your always fairly safe installing TTF and/or OTF fonts. Also, many websites will offer only Windows or Mac downloads. In those cases (without going into an even longer explanation), it is usually easier for the Lubuntu user to choose the Windows version.