fetch is used to ....wait for it...fetch files from an URL, either by http or ftp. The man page will give you all the details you won't want to know about if you don't need to do weird scripting stuff with fetch, but there's a few that might be good to know off the bat:
-a retry after soft failures -r restart previously interrupted transfer -F force -m mirror -- if the same file (with same size) already exists, don't fetch it -p passive FTP -d direct connect; ignore proxy even if configured -f specify local filename
Typically, you'll want to specify the protocol in the URL itself - for example
ph34r# fetch http://www.freebsdwiki.net/index.php\?title=Fetch -f ./Fetch.html
to grab a copy of this page and save it as Fetch.html in the current directory.
If you're coming to BSD from the land of Linux, fetch is very similar to wget.