No one can exactly say how many movie genres there are nowadays. In one thing you can be sure ' there are more of them than one can think. There still remains the classic system of genres that started to form with the appearance of the first movies.
But the borders between different movie genres are very blurred and ones of them slightly flow into the others forming some intermediate genre forms. Some new alternative movies can't be even named by any of all genres. Filmmakers more and more often search for some new forms.
Main questions about movie genres are whether one should follow the traditions of a chosen genre while making the movie. The there question is whether some of movie genres are more commercial than other, and if they are then what are they.
Every period in movie history has it's own list of the genres which are more popular at the moment. But now the variety of the movies produced all other the world is so really great that it makes the situation on the genre field too changeable and elusive. So we can never tell which one is the leading.
Still let's name some of those classical genre names that can be seen and heard almost everywhere. Those genres form a kind of a signal system for a viewer. You read about the movie that it's for example 'horror' and you already know what to expect from it and even whether you will or won't watch it.
What are those general expectation from different genres?
Comedies. It's definitely the most positive genre. It makes us laugh and as we all know laughter prolongs our lives. Comedies are dearly loved by most of all people and it's no surprise. They usually don't speak of any serious problems. Comedies are based on some situations when people get in some silly and embarrassing situations, switching their social or any other roles, when everything gets head over heels. So when we see 'comedy' marked under the movie title we usually know that this means two hours of the good mood and loud laughter for us.
Action movies. Well they are filled with action. Everything is in motion in these movies: people are running, fighting, things are happening, cars are racing, bombs are blowing up, situation is changing every moment. These movies won't let you fall asleep, you will probably feel giddy one moment but you will never feel bored. Action movies are filled with special effects and very spectacular, especially if their budget exceeds the figure of a few tens of millions dollars. So 'action' means usually that you should comfort yourself and enjoy the show.
Dramas. These are life stories, more realistic than any other movie genres (except documentary). You watch people both common or some famous personalities going through their trials and tribulations of all the kind. It's not an entertaining genre, it comes closes to art. And it will require you some mental effort to watch it. Good dramas get Oscars and make people thing. It's not the type of movie that will help one to relax.
Melodramas. If dramas tell us about life then melodramas usually tell some happy or sad love stories. Sometimes they are called 'romantic'. Melodramas appeal to our most sentimental emotions. There's a very popular advice for the men taking their ladies on the first date to take them to the cinema where's some melodramatic movie is shown. If a lady starts crying at the most sentimental moment a man will get a chance to console her and show some care.
Horrors. They do exploit our feelings of fear which we naturally experience at the face of the terrible, unknown, evil, mysterious, dangerous and etc things. In the basics of a horror lays unknown but expected danger usually something supernatural. They say through the sharp fear we get catharsis. The fans of an adrenaline rush will probably agree with this.
Thrillers. It shares some traits of a classical detective and a horror. Unlike the horror it's not a 'get me scared' genre, unlike the detective it gets you all trembling while some mystery is being solved. Both your brains and your nerves are being engaged while watching. This movie gets you thrilled without much special effects, scary creatures or dark forces involved but only by the mean of some psychological effects.
These are very few from the general amount of all movie genres. There are also detectives, fantasies, westerns, fantastic, family, documentary, sci-fi, animated movies and etc plus some more specified genres. And mind that almost every movie contains the features of a few different genre so one can never stop talking about movie genres.