EM is a general-purpose library for nonblocking IO. Tornado is an HTTP server. In as much as they're both about nonblocking IO, they are similar.
Tornado has its own EventMachine-like equivalent as part of the stack they released. So you could use it to do TCP non-blocking IO if I grokked the code correctly.
I'd say it's a little more similar to Thin, which is a web server based on EM.