Comma-separated values: a rudimentary plain-text spreadsheet. Also includes tab, space, semicolon or other-separated values.
Comma-separated values are a widely-supported and easy-to-parse rudimentary plain-text spreadsheet which is commonly used for exchanging data between programs.
In general, data is separated by a delimiter (comma, pipe |, semi-colon ;, etc..) that will trigger a break in the data by the application that is reading the file. The data is then parsed into the groupings native for the application whether it be cells on a spreadsheet or boxes in a form. The widely-supported nature of .csv files make them extremely versatile.
Additionally, there are specific types of csv files which will dictate what delimiter is used (windows csv, ms-dos csv, etc).