In recent years, the Internet speed, the increase of personal storage, and the appearance of numerous applications, sites, and platforms that provide and facilitate access to a huge amount of music, have reached new heights. These technological advances, not only have given to a simple user unlimited access to music but have also made convenient the procedures of recording and publishing personally created music to the web.\newline

This great amount of music data existing, that year after year is increasing, necessitates organization. As a level of comparison, to understand the amount of music data, in 2018, the music streaming service \textit{Spotify} provided access to more than 35 million songs \cite{spotify},
while the collaborative metadata database of musical releases \textit{Rate Your Music}, contains data of 1,228,075 Artists \cite{rateyourmusic}.
Of course, this amount of musical pieces is not the real existing, as many artists and/or albums/songs do not appear in these services. Nowadays everyone can compose music, upload and promote it into numerous platforms. From well-known sites targeted for this reason, to personal sites and social media.\newline

This large amount of data needs to be organized and be described symbolically.
After the maybe most important metadata, which is the artist or creator that released the music, the descriptor used is the music genre it belongs to. Music genre defined by Jim Samson \cite{jimsamson} is:\newline

” \textit{A class, type or category, sanctioned by convention. Since conventional
definitions derive (inductively) from concrete particulars, such as musical works or musical practices, and are therefore subject to change, a genre is probably closer to an “ideal type” (in Max Weber’s sense) than to a Platonic “ideal” form. Genres are based on the principle of repetition. They codify past repetitions, and they invite future repetitions.} “\newline

Genre is a form of classification. It is a way of describing what a music piece shares with others, and what differs it from others. As a result, music genre identifies pieces of music that share some characteristics. The use of subdivision of a genre (subgenre) is also usual. According to Aucouturier and Pachet \cite{Aucouturier} “The genesis of genre is therefore to be found in our natural and irrepressible tendency to classify”.\newline

Music genre existence is important for many reasons. Genre is the way that one navigates through the vast amount of music existed, and people usually listen to new music according to this classification, as songs that belong to the same genre is more probable to be liked by them. Also, we identify what music we listen to by its genre, or we organize our music according to it. Furthermore, record companies use these labels in artists and albums to target a particular type of audience or age group.\newline

Nevertheless, because genre classification is subjective and has an ambiguity in its nature, is sometimes controversial. Some \cite{econotimes} support that music genre classification is outdated, and suggest classification based on the three following categories: “Arousal” (the energy level of the music), “Valence” (the spectrum from sad to happy emotions in the music) and “Depth” (the amount of sophistication and emotional depth in the music). \newline

This ambiguity is what makes music genre classification hard. According to Aucouturier and Pachet \cite{Aucouturier}, genre may be used as an intentional or as an extensional concept.
In the first case, genre has more subjective characteristics, and relies on “collective cultural knowledge”. In this category fall music genres that refer to social, historical or geographical context.
On the other hand, genre may be used as an extensional concept. In this view, genre is a dimension of the music piece itself, and characteristics like tempo, timbre, pitch, or the language of the lyrics define it.
These two concepts do not always agree, when used to characterize the same music piece.\newline

Furthermore, it is incorrect to classify a song according to its artist’s previous work. It is usual for many artists to compose music that belongs to a different genre in a different album, but also tracks in the same album can belong to different genres or subgenres. Also, a song itself can have elements that belong to many genres/subgenres. \newline

In addition, with the progression of music, new genres appear. These can be either distinct to the ones existed, or are influenced by preexisting ones.

In general, experienced humans can classify audio pieces in genres and subgenres. Usually, a simple listener is unable to categorize music into a genre or subgenre if not familiar enough with.\newline

While music genre classification can be assigned to music experts that can identify the audio features and characteristics needed for the categorization, the huge amount of music existing and composing continuously makes it an almost impossible task.
For this reason, automatic music genre classification has become a necessity. Music genre classification is a part of the area of Music Information Retrieval (MIR), along with automatic music transcription, automatic music generation and instrument recognition.