There are classic old style fonts, such as Goudy Old Style, which were designed in the 20th century. The name ``old style'' refers to the style of the font, as opposed to the date of its design. They are well suited to writing long documents. Old style fonts tend to be conservative in design, and very readable. Old style fonts are based on very traditional styles dating as far back as the late 15th century. It is somewhat overused, and many books by typographers plead users to stay away from it.
By the way, Helvetica is considered harmful by typographers. Notable sans serif fonts include Lucida Sans, MS Comic Sans, Verdana, Myriad, Avant Garde, Arial, Century Gothic and Helvetica. For example, Microsoft touts Verdana as being readable at very small sizes on screen.
The lack of detail in the font can provide it with more clarity. Another application that sans serif fonts have is as display fonts on computer screens, especially at small sizes. There are sans serif fonts that are readable enough to be well suited to documents that are supposed to be browsed / skimmed (web pages, catalogues, marketing brochures). One usually does not write a long book using a sans serif font for the body text. Sans serif fonts do not have these little hooks, so they have a starker appearance. There are many different types of serif fonts. Serif fonts are usually considered more readable than fonts without serifs. For example, the letter i in a font such as Times Roman has serifs protruding from the base of the i and the head of the i. Serifs are little hooks on the ends of characters.