Here is a list that will help you understand what each Genre stands for.
Action/Adventure : primarily focuses on battles, war, and physical competition. Martial arts, weapon fighting, or other action oriented material are usually featured.
Examples: Naruto or One Piece.
Fantasy : the characters use magic and other supernatural elements/forms as a primary element of plot of the series.
Examples: Naruto or Kyou Kara Maou
Drama : primarily has a high degree of character development and emotional themes. Many center around relationship complications.
Examples: Fushigi Yūgi or InuYasha.
Comedy : the main character or the ones around him will often end up in funny accidents in order i to provoke laughter amongst the audience.
Examples: Ninja Nosense or Goshuushou-sama Ninomiya-kun
Mystery : no understanding of the full plot of the series until the end.
Romance : dwelling on the elements of romantic love.
Examples: Love Hina or Shuffle!
Game Based : primarily revolves around some sort of card or board game.
Examples: Yu-Gi-Oh! or Hikaru No Go.
Horror : use darker story and/or supernatural themes.
Examples: Vampire Hunter D series or Chrono Crusade.
Science fiction : consists of futuristic elements, particularly featuring future science and technology. They can be drawn from current scientific studies and inventions or created to meet the needs of the world.
Examples: Ghost in the Shell or Wings of Honneamise
Bishōjo : is Japanese for "beautiful girl". A blanket term that features pretty girl characters. Sometimes conflated with Moè.
Examples: Magic Knight Rayearth or Negima.
Bishōnen : is Japanese for "beautiful boy". A blanket term that can be used to describe any anime that features "pretty" and elegant boys and men.
Examples: Fushigi Yūgi or most CLAMP shows.
Sentai : is literally a "fighting team" in Japanese. It refers to any show that involves a superhero team.
Examples: Cyborg 009 or Voltron.
Robot/Mecha : features super robots.
Examples: Mobile Suit Gundam or Mazinger Z.
Post-Apocalyptic simply deals with a post-apocalyptic world.
Examples: Fist of the North Star or Nausicaä of the Valley of the Wind.
Moé : features characters with perky, cute, weak, or naïve behaviors. In some way, they are not overly independent.
Examples: A Little Snow Fairy Sugar.
Harem : is a genre which focuses on a male character surrounded by the romance of multiple female characters. Typically, the male cohabits with at least one female.It is usually marketed as a Shōnen or Seinen.
Examples: Ranma ½ or Love Hina.
Reverse Harem : reverses the gender balance in harem, where a female character is romantically involved with many male characters. It is more often than not a Shōjo or Josei Anime.
Examples: Ouran High School Host Club or Fruits Basket.
Magical Girlfriend : is more accurately termed Exotic Girlfriend. This genre focuses on the romantic relationship (and cohabitation) between a man and at least one woman of extraordinary origins such as alien (Tenchi Muyo!, Urusei Yatsura), supernatural (Oh My Goddess!), or technological (Chobits). Often considered a subgenre of Harem.
Ecchi : is Japanese for "indecent sexuality", derived from the Japanese pronunciation of the letter "H". Sexual humor and fan service are prevalent.
Examples: Dears or He Is My Master.
Hentai : is Japanese for "abnormal" or "perverted". This term is synonymous to pornography or erotica, as hentai content specifically consists of such.
Examples: La Blue Girl or Bible Black.
Shōjo-ai or Yuri : is Japanese for "girl-love". These focus on love and romance between female characters. It is often being replaced by the term "Girls Love" (GL). Yuri is like Shōjo-ai, but sometimes involves older characters or explicit sexual activity.
Examples: Revolutionary Girl Utena or Kannazuki no Miko.
Shōnen-ai : is Japanese for 'boy-love'. These focus on love and romance between male characters. The term "Shōnen-ai" is being phased out in Japan due to its other meaning of pederasty, and is being replaced by the term "Boys Love" (BL).
Examples: Loveless or Gravitation
Tell me if i miss any Genre, some genres i left out like Lolita.. no need for people here to know that kind of thing..
I hope this comes to use because it is good to know what kind of anime it is befor you download it
Example: Naruto genre : Action, Fantasy
in other words the characters use magic or other supernatural abilitys and elements. rimarily focuses on battles, war, and physical competition. Martial arts, weapon fighting.