Search Engine Marketing, this includes other sorts of online marketing such as Email campaigns.
SEM = outgoing/outbound/pushing for traffic/inbound marketing – Actively advertise your site via various channels such as social networks or Adverts. This is an active process by you.
SEO = aquire incoming traffic / getting them to coming to you / improve the semantic and relevance of the content for search engines to improve your ranking. This is almost a passive process as you cannot control the search engines.
Analytics is the art of reading incoming traffic for patterns to help you improve your web presense and is used as part of SEO and SEM for feedback.