Is silencer the same as gene silencing? I know that gene silencing refers to those heterochromatin concentrated at the telomeres or centromere. It is also related to methylation. But what about a silencer. Are they the same? Or is silencer an opposite of enhancer (repressor)?
You are a bit confused with your terms here. Remember, when you are talking about natural regulation of a transcriptome, you can control it at the level of the genome, transcriptome and the epigenome.
The Repressor protein is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. Depending on where it binds, it regulates at the level of the genome or transcriptome.
The Gene Silencer Region, is a DNA sequence in the geneome that that allows repressor proteins to bind to it. It is usually very near the promoter of the gene that it wants to control. This controls the expression of the gene at the level of the genome to repress the transcription of the gene. There exists a few types (Classc silencers, negative regulators and polycomb response) but lets not get into that).
An example is the regulatory element in the classical Lac operon.
Gene Silencing as you call it is epigenetic in nature. I'll point you towards the wiki as it is quite big and diverse to explain it here: http://en.wikipedia.org/wiki/Gene_silencing
you can read more about it here:http://www.ncbi.nlm.nih.gov/books/NBK26872/