The MARC 505 field is used to describe the contents and the MARC 520 is used for a summary of the item.  Both fields are very useful to not only see what an item is about but to have the item displayed in search results in the first place. However, TOO much information can be a bad thing! It can cause errors while cataloging and patrons are not likely to read a long paragraph of data. So limit summaries to 2 or 3 sentences.

MARC 505 –  Titles of parts or the table of contents and possibly statements of responsibility.

Example of basic formatting:
505 0# $a Carbon —  Nitrogen —  Sulphur —  Metals.

Example of enhanced formatting (RDA Preferred) (t-title of chapter, r-author):
505 00 $t Quark models / $r J. Rosner —$t Introduction to gauge theories /$r C. Quigg —$t Deep inelastic scattering / $r D.H. Perkins —$t Jet phenomena /$rM. Jacob

For more information: http://www.loc.gov/marc/bibliographic/bd505.html

MARC 520 -Unformatted information that describes the scope and general contents of the materials.This could be a summary, abstract, annotation, review, or only a phrase describing the material.The level of detail appropriate in a summary may vary depending on the audience for a particular product.

Example:
520 ## $a Kate and Ben follow their rabbit into a haunted house and discover the source of the house’s ghostly sound.  While Scooby waits nervously in the van Kate and Ben set the ghost free and bring their rabbit home safely.

For more information:  http://www.loc.gov/marc/bibliographic/bd520.html