Jack is correct.
ANY certificate, even a self-signed certificate, on the web server will provide encrypted transmission of data.
The purpose of a certificate is not encryption. The purpose of a certificate is to provide trust.
You go to
www.somedomain.com. We all know that the Internet can be fragile, and though things usually work fine, there's really no absolute guarantee that the website that answers your browser really is, in fact,
www.somedomain.com. It could be
www.hackers-r-us.com pretending to be
www.somedomain.com.
A certificate can solve this problem.
There are companies out there (Verisign, Thawte, etc) that issue certificates to clients based on their own internal policies. In order to get a cert from, say, Verisign that says you're
www.somedomain.com, Verisign has internal policies that require you to prove that you really do own
www.somedomain.com before they'll issue you a certificate. What you're paying for is the level of trust that those companies provide through their screening processes.
The certificate, when your browser gets it from the web server, says, "this web server is
www.somedomain.com, and Verisign says so!" Your browser can verify that the cert actually was issued by Verisign, and that the cert was issued to
www.somedomain.com. That provides a high level of trust to your clients.
With a self-signed certificate, your browser gets a cert that says, "this web server is
www.somedomain.com, and Joe Schmoe says so!" Your browser probably doesn't have Joe Schmoe's certificatation authority as a trusted certification authority. Your browser will then display a dialog saying that the site has a certificate, but that the certificate issuer is not trusted.
At that point, it's up to the user to determine whether or not to trust a site backed by a certificate from Joe Schmoe.
Note that a certificate from Verisign doesn't mean the website is trusted or trust-able. It just means that you can trust that they are who they say they are, because Verisign verified that before issuing the cert. (Even then, mistakes happened. Verisign once issued a cert with Microsoft's name on it to a non-Microsoft entity with criminal intent.)
So, all the cert does is say, "the entity that owns the web server passed the certificate issuer's requirements for being who they say they are." Some certificate issuers have a very low bar for that test.