Session ID
Definition
A session ID is an identifier used to recognize multiple visits of the same user and comprise them to a single session. At the beginning of a session, the server generates a unique ID and sends it to the visitor's browser. The browser will then send this ID back to the server each time he communicates with him (e.g. when the visitor opens another page).
If the browser doesn't send a session ID to the server, the server will consider it a new session and generate a new session ID.
The server uses this session IDs to assign information stored on the server (e.g. the shopping cart content) to the same visitor each time he visits the website.
Importance for SEO
If possible, you should avoid using session IDs because they can confuse crawlers: Each time the crawler visits the website, he will see the same content under another URL (because of the different session ID suffix). The crawler could think it's another page with the same content and consider it duplicate content.
If you have to use session IDs, some search engines (e.g. Bing, Google) offer the possibility to define URL parameters that should be ignored by crawlers.
