Most of the web hosting companies offer packages with unlimited resources such as disk storage, bandwidth, MySQL databases, email address, FTP account and so on. Also, they offer these resource for a few pounds monthly. However, this is not really possible. It is actually a method to increase sales based on a fake promise.Shared Hosting Unlimited HostingOverselling
The concept of unlimited disk usage, bandwidth, resources is that you can as much as disk space, bandwidth, resources. You won't get any warning message or alerts if you will use it in GBs or to any limit. This is the concept of the unlimited web hosting plan. But every company has defined it in their own way. You should clear the actual meaning of unlimited plan before sing-up.
There is some limit for the unlimited plan, no doubt but it's for the server administrator. You don't have any limit for the usage.
The company offers unlimited web hosting in fact usually using limits system on per user basis. For example if you have a wordpress website in your unlimited hosting account it will be stored on a node-server in isolated hosting account. And webhosting companies setup a limitation like 256 Mb RAM, so you can not request much RAM on the server. Another technology to host on a single server as much customers as possible. This also calling overselling. These two tricks allow company to offer 'unlimited' hosting.
well though its a fake promise there are TOS and AUPs at the end of the website providing unlimited hosting and stuff.. when you are signing up for unlimited hosting you can not upload your private stuff such as PDFs , MP3s , backups etc.. to the hosting space , they will simply remove it by calling them as non web related contents so the space you get is dedicately for your website files and emails only.