what is the differnce between SKOS, RDFS and OWL?

Google is your friend... there's tons of documentation out there (example)

A short intro, from what I understand now:
RDFS is the basis of RDF data modeling : with rdfs:Resource, rdfs:Class and rdfs:Literal you can start your own ontology.
OWL added on top of that more logic and reasoning capabilities, but it has basically the role as RDFS : to provide building blocks for creating vocabularies. Think of it like RDFS evolved.
Owl:Class, Owl:ObjectProperty are sub-classes of their RDFS counterparts.

SKOS is just a stand-alone vocabulary, built with OWL and RFS, like many others like FOAF, SIOC, GoodRelations...
Its specific purpose is to model Thesaurii in RDF : you can easily map concepts trees and relations using the SKOS vocabulary. It's being widely used beyond the librarian's world, partly because of its better labelling features (prefLabel, altLabel...) that can be used with any kind of real-world data. SKOS-XL extends this labelling features by giving a Class to the Label object

The answer of dguardiola is definitely THE ANSWER, i would just add these:

So, first you have RDF which uses XML syntax to implement a semantic to describe resources. But it has limited vocabulary.

Then, you have RDFS, which is built on top of RDF with some stronger semantics such as Class,SubClass,Domain,Range

Now, you have SKOS and OWL which have different semantics and are intended for different application. OWL provides a language for knowledge representation and has semantically strict relationship such as Super/Subclass relationship.

SKOS is used mainly for knowledge Organization and has weaker semantic relationship such as Narrower/Broader,related and lexical relationship for prefLabel(prefered label),altLabel(alternative Label). OWL is very expressive and such expressiveness is not desired in some application because expressiveness do has a cost. So, SKOS has limited expressivity compared to OWL. It can be used for e.g. in Document Navigation System while OWL will be used in areas where lots of inferences need to made to get new piece of knowledge.

The broader/Narrower in SKOS should not be confused with Super/Subclass in OWL. In SKOS, if a concept A is linked by "broader" to concept B, it doesn't mean than B is the superclass of A and we cannot make inference that all As are B.