It means two things.
One, it's sort of a joke! We're poking a little fun at our industry's never-ending "arms race" to offer more and more disk and bandwidth.
For more than a decade, it's been like:
Other Hosts: "10MB"
DreamHost: "20MB"
Other Hosts: "50MB!"
DreamHost: "100MB!"
Other Hosts: "500MB!!"
DreamHost: "1GB!!"
Other Hosts: "100GB!!!"
DreamHost: "500GB!!!"
And finally:
Other Hosts: "UNLIMITED GB!!!!"
.... to which we had no choice but to reply ....
"UNLIMITED + 50GB!!!!"
Two, it's a reference to our policy of allowing 50GB of space for backups use free, with extra at only 10 cents a GB / month. (The rest of your disk space may only be used by files needed for your websites directly.) No other "unlimited" host allows using any of your disk space for backups, at all!
What we mean is, you don't have to worry about disk storage or network transfer when your site gets popular.
You DO still have to worry about plenty of other things though... if your site isn't well optimized and is causing issues for others on your shared server (either because it's pounding the CPU, hogging RAM, or doing tons of disk I/O), you may be asked to sign up for your own DreamHost Private Server.
Trust us, every host out there has exactly the same limitations.. we're just trying to be up front about them!
Basically, sites whose essential purpose is to use disk or bandwidth.
When making a website, you should be thinking about "How can I make an interesting site for my visitors while minimizing my server storage, bandwidth, file system, memory, and cpu impact as much as possible?"
The result will be a better experience for your visitors, your web host, and yourself!
Here are some specific examples of things not allowed:
Actually, for you, Mr. Legitimate Website Owner, it is unlimited.
This whole page is just to announce to the <.1% of people who want to try and run free porn affiliate sites or anime video trading forums or host pirated rips of The Dark Knight on DreamHost that we're not having any.
But our pledge to you is to do our best to always keep your website, with the content you created, UP... no matter how many times you get slashdotted, dugg, or win the DHSOTM!