robots.txt logs: results?

I’ve discovered that msnbot has crawled my site. I wanted to know the results, I don’t understand it too well, is the “…404.. ” an error code?:

[QUOTE]

65.54.188.66 – – [25/Aug/2004:13:03:52 -0400] “GET /robots.txt HTTP/1.0” 404 640 “-” “msnbot/0.11 (+[url]http://search.msn.com/msnbot.htm[/url])”
65.54.188.66 – – [25/Aug/2004:13:03:53 -0400] “GET /javascript/inputwrite.html HTTP/1.0” 404 640 “-” “msnbot/0.11 (+[url]http://search.msn.com/msnbot.htm[/url])”

[/QUOTE]

The IP I understand, the time stamp I understand, what does “…

“GET /javascript/inputwrite.html HTTP/1.0” 404 640 “-” “msnbot/0.11

…” mean?
and obviously the (+[url]http://search.msn.com/msnbot.htm[/url]) is the crawler.
In any case, this means that a crawler is trying to index my site?
Also, do I need to have the robots.txt file?

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