A keyword is a significant word in the description of a topic for which you are searching.
It describes the topic fully or describes an important aspect of the topic.
Let's say you want to find out about - "how you go about downloading music for an ipod."
This phrase has 9 words. The most significant of these are:
These would be three useful keywords when searching for information on this topic.
How, you, go and about are not useful keywords because they do not add information to the three keywords already chosen.
Words like for and an are never keywords: they are simply in the topic description to make it grammatically correct.
Keywords are usually nouns - music, ipod - or verbs - download