AIS receivers are the lowest cost units. They will show you where other suitably equipped vessels are relative to your position on your existing suitable/compatible screen. Suitable screen are - depending on receiver model selected - Plotter, PC, MAC, iPhone, iPad, iTouch etc. BUT you really should be considering the move to a Transponder - that transmitts your position to other vessels as well, not just receives. The cost difference is no longer large. Some countries (eg SIngapore) will no longer allow you in their territorial waters without an AIS Transponder.