First, the good news: If you have a website, then you have a site that can be accessed by any mobile device with a browser.
Now, the bad: Chances are, that site looks pretty crappy on said mobile device.
A site designed to be viewed on a desktop PC comes across as woefully lacking. Say you’re accessing such a site from the Safari browser on your iPhone. The first thing you’re likely to notice is that it takes a relatively long time to load. The second thing is that the type on the page is pretty small. It might take a lot of zooming and pinching to navigate the site as well. If you have Flash on your site, it’s not going to come across at all on an iPhone.
A responsive or mobile website is just a website that has been optimized for mobile browsing. It can be reached through a phone's web browser directly.
There are some great advantages to doing a responsive or mobile site: