Software in this category is used to search game files and extract assets (such as sound effects and music) so they can be more easily accessed. Rippers often work by performing a brute-force search, which means they often need little or no knowledge of the data files being searched. The only drawback is that only known file formats can be extracted. If a game uses obscure or custom music formats for example, it often won't be possible to extract the music with a ripper. It is also very easy to thwart a ripper, by compressing or encrypting part of the file. For example, later versions of Blood encrypted the first 256 bytes of each file inside the RFF archives, which prevented automatic extraction by ripping utilities.
Typically programs that are aware of the underlying file format and extract files based on an index (regardless of their content) are called extractors rather than rippers. For example an unzip utility, which works regardless of what format the files are inside the .zip, is an extractor.