[ RadSafe ] CTBTO press release....clarification/question

[Miller, Mark L]

 The way I read it, the U.S. has signed the treaty, but NOT ratified it.  What's the distinction?

Background:
180 States, the vast majority of the international community, have signed the CTBT, underscoring their support for a definitive ban on nuclear testing. 148 of these have also ratified the Treaty. To enter into force, however, the CTBT must be signed and ratified by 44 specific States. These States participated in the negotiations of the Treaty in 1996 and possessed nuclear power or research reactors at the time. Thirty-five of these States have ratified the Treaty, including the three nuclear weapon States France, Russian Federation and the United Kingdom. Of the nine remaining States, China, Egypt, Indonesia, Iran, Israel and the United States have signed the Treaty, whereas the DPRK, India and Pakistan have not yet signed it.

CTBTO press release
http://www.ctbto.org/press-centre/highlights/2009/ctbtos-initial-findings-on-the-dprks-2009-announced-nuclear-test/