The intension of a word or phrase (not to be confused with intention or intentionality) may be regarded as a description, or set of properties (if one can indulge talk of properties at all), which applies to each member of the word's extension, and which distinguishes those things from everything not in the extension. In other words, a word or phrase's intension is the set of necessary and sufficient conditions for identifying what is in its extension.
This is a rather complicated definition, but it is not a difficult concept. What, for example, would be the intension of the phrase 'Wikipedia readers'? Simply the property of having read any part of Wikipedia at any time. Anything that has that property is part of the extension of the phrase 'Wikipedia reader'. For example, Jimbo Wales has been reading Wikipedia; so the intension of the phrase 'Wikipedia reader' applies to Jimbo Wales; and so Jimbo Wales is part of the extension of 'Wikipedia reader'. Moreover, anything that has not read any part of Wikipedia at any time is for that reason not part of the extension of the phrase 'Wikipedia reader'.
Discussions of "intension" often get significantly more complicated than the above. Many philosopherss and semanticists pursue the topic nonetheless, believing that it gets to the heart of the matter in questions about meaning and content.
In Computer Science, some database textbooks use the term intension to refer to the schema of a database, and extension to refer to particular instances of a database.