The subject indexing of music is not only performed by professionals in the music industry, among which we can include musicians, producers, sound engineers, music editors, and music librarians; there are also non-professionals which include ordinary listeners and among them those that are especially familiar with a particular musical genre. When subject indexing of music is done by ordinary listeners, it is usually done by social tags or labels through the process commonly known as collaborative tagging. In order to be able to understand what these social tags mean, this work will explain the types and tasks of social tags. Special attention will be given to problems that arise when describing music with social tags in network systems for playback, setup, and when searching for musical content. The theoretical part of this paper encompasses the basic theory of music, the peculiarities of applying the Universal Decimal Classification for music collections, ways of marking musical genres, the definition of social tags and how to use them as recommendations. This is followed by the research part of the work. A survey was conducted through two Google Forms surveys. 80 respondents took part in the surveys, of which 43 respondents took part in the first survey, and 37 of them took part in the second survey. The first survey was used to explore how respondents describe the offered songs using the suggested tags, while the second survey used the same song but without the proposed tags. Furthermore, it was investigated whether the respondents would mostly use the genre category in both surveys. The results showed that respondents, when given description tags of songs would use the genre category more often, and when they would assign the tags themselves, they would choose the mood category.