What to put in it
The "/robots.txt" file is a text file, with one or more records. Usually contains a single record looking like this:

User-agent: *
Disallow: /cgi-bin/
Disallow: /tmp/
Disallow: /~joe/

In this example, three directories are excluded.

Note that you need a separate "Disallow" line for every URL prefix you want to exclude -- you cannot say "Disallow: /cgi-bin/ /tmp/" on a single line. Also, you may not have blank lines in a record, as they are used to delimit multiple records.

Note also that globbing and regular expression are not supported in either the User-agent or Disallow lines. The '*' in the User-agent field is a special value meaning "any robot". Specifically, you cannot have lines like "User-agent: *bot*", "Disallow: /tmp/*" or "Disallow: *.gif".

What you want to exclude depends on your server. Everything not explicitly disallowed is considered fair game to retrieve. Here follow some examples:
To exclude all robots from the entire server

User-agent: *
Disallow: /


To allow all robots complete access

User-agent: *
Disallow:

(or just create an empty "/robots.txt" file, or don't use one at all)
To exclude all robots from part of the server

User-agent: *
Disallow: /cgi-bin/
Disallow: /tmp/
Disallow: /junk/

To exclude a single robot

User-agent: BadBot
Disallow: /

To allow a single robot

User-agent: Google
Disallow:

User-agent: *
Disallow: /

To exclude all files except one
This is currently a bit awkward, as there is no "Allow" field. The easy way is to put all files to be disallowed into a separate directory, say "stuff", and leave the one file in the level above this directory:

User-agent: *
Disallow: /~joe/stuff/

Alternatively you can explicitly disallow all disallowed pages:

User-agent: *
Disallow: /~joe/junk.html
Disallow: /~joe/foo.html
Disallow: /~joe/bar.html