Wel... meta-tags are header tags for providing meta-data of the web page. These are important because search engines trust some of this information when crawling and "storing" the web. Which and how search engines use this information variates from one to another. Some of this meta-tags are:

<META NAME="author" CONTENT="">
<META NAME="subject" CONTENT="">
<META NAME="Description" CONTENT="">
<META NAME="Classification" CONTENT="">
<META NAME="Keywords" CONTENT="">
<META NAME="Geography" CONTENT="">
<META NAME="Language" CONTENT="">
<META HTTP-EQUIV="Expires" CONTENT="">
<META NAME="Revisit-After" CONTENT="">
<META NAME="distribution" CONTENT="Global">
<META NAME="Robots" CONTENT="INDEX,FOLLOW">
<META NAME="country" CONTENT="">

Hope it helps.