I would like to know what is the difference between probe sets and tags. I know that a probe set is a collection of probes, for example 12345_at, that would refer to a gene that I want to know if its expressed? and what about tags? are they different? Thanks
Tags typically refer to short, discrete sequences that are gathered from a pool of larger sequences (like total RNA). These are sometimes used to measure gene expression using counts of these "tags" rather than fluorescence intensity on an array.
A probe (and probe set) is designed to "fish" what you're interested in (the target), whereas a tag (artificially binded to the molecule) will help you to identify the origin of the molecule before the hybridization step.
If you want to read up about tags search for "SAGE tags" (SAGE stands for serial analysis of gene expression).
You can start at: http://en.wikipedia.org/wiki/Serial_analysis_of_gene_expression
Note that the tags are created by digesting (see: e.g. http://mkweb.bcgsc.ca/perlworkshop/data/courses/0.1.0.1/01/pdf/0.1.0.1.1a.a1.pdf) not by adding something and the analysis is normally done by sequencing not by hybridizing.
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I think the tags you refer to here is what we usually refer to as "the label", although I agree that labeling is a kind of tagging as well that is different from what we usually refer (or referred, it is not often used anymore) as tagging in gene expression tagging. See my answer on SAGE approaches.
I tried to suggest a most generic as possible definition. However, I hope I did not introduce any ambiguity. I agree with your comment.