If you know a bit of French already, you might think that the way you've learned French is the way people speak it, which is a fair assumption. However, this is not always the case because the French you learn is just one type of French, which is the standard French. Languages are usually divided in three registres - levels: regsitre familier, registre courant and registre soutenu.
1. Registre courant.
This is what we call standard French and the language that is used on an everyday basis and with anyone. This is also the type of French we learn at school (both in France and abroad) and the French you can hear in films, series or the media in general. The pronunciation is correct, the vocabulary clear and simple and the grammar is respected - note that we do not use tenses such as "passé simple" or "imparfait du subjonctif" in the registre courant.
2. Registre soutenu.
This registre is a lot more complicated than the registre courant as it is a more literary level of the language. If you have read books written by famous French authors, such as Victor Hugo, Maupassant, Flaubert, Zola etc. you have already come across this kind of French. This language is used mostly in writing although some very rich, posh or scholarly people might use it verbally as well. The pronunciation is meticulous, the vocabulary is rare and researched with a lot of cultural input and the grammar will include rare tenses such as le conditionnel passé, l'imparfait du subjonctif ... Sentences will be longer and carefully thought about.
3. Registre familier.
This is the registre you will most often hear on the streets and amongst friends and family. Some syllables or even full words are not pronounced (such as the ne in ne...pas), expressions and vulgar words are used and the grammar is not always correctly used and rules are easily ignored. Vulgar words and expressions are part of everyday life in France and it is not surprising to even hear some at work on on the medias.I am sure that you already know the most used vulgar words in French: merde and putain!
3 bis. Le Verlan!
You might not know this one... Le verlan is very often used in French and dates back from 1960s. It has evolved since then and is still changing today but it's part of the everyday use of the language. The verlan, which comes from "a l'envers" is the fact to swap syllables around: envers - vers en - verlan! Let's go through a few examples you can hear on an everyday basis:
1. "T'as maté ce film? Il est ouf!" = "Tu as regardé ce film? Il est fou".
fou = crazy
ouf (verlan from fou) = great, surprising, crazy (depending on context).
maté = regardé (familier).
2. "Il est hyper chelou ce mec, t'as vu comment il m'a téma" = "Il est tres bizarre cet homme, tu as vu la maniere dont il m'a regardé?"
hyper = tres (very)
chelou (verlan from louche) = bizarre, weird
mec = guy
téma (verlan from maté) = regarder
3. "C'est mort, je fais pas l'exo en plus le prof est grave relou, il m'a trop saoulé" = "C'est hors de question, je ne ferai pas l'exercice en plus le professeur est casse-pied, il m'a agacé"
c'est mort = non (no way)
exo = exercice
prof = professeur
grave = tres (very)
relou (verlan from lourd) = casse-pied, agacant (annoying)
saoulé = agacé (annoyed).
When you start learning French, you will learn the everyday basic French which will be part of the registre courant. This is the French you will use most often and will need to be more familiar with. The registre soutenu is not necessarily something that you need to know - if you are interested in literature, philosophy or something more scholar then yes - otherwise, you can probably skip it altogether. The registre familier is very useful for someone who wants to live, study or work in France and wishes to have french friends as it will give you the knowledge to speak in a more casual and authentic manner. However, you will need a certain knowledge of the registre courant before moving on to the registre familier as otherwise it will only confuse you and you don't want to sound rude by using the wrong level of French language!