Semantic annotation, which is considered one of the semantic web applicative aspects, has been adopted by researchers from different communities as a paramount solution that improves searching and retrieval of information by promoting the richness of the content. However, researchers are facing challenges concerning both the quality and the relevance of the semantic annotations attached to the annotated document against its content as well as its semantics, without ignoring those regarding automation process which is supposed to ensure an optimal system for information indexing and retrieval. In this article, we will introduce the semantic annotation concept by presenting a state of the art including definitions, features and a classification of annotation systems. Systems and proposed approaches in the field will be cited, as well as a study of some existing annotation tools. This study will also pinpoint various problems and limitations related to the annotation in order to offer solutions for our future work.