Choosing a web host is really a very difficult task but choosing the right Web Hosting service for your website can be a little bit confusing. There's so many choices out there and many people aren't exactly sure what they should be looking for.
The disk space depends upon on how many files you plan to upload to your hosting space. If it's a personal or ecommerce site with just a few pages and graphics, than 25 MB of disk space a month should be more than enough. If it's a multi-page site such as an online shopping mall, then 2000 MB would be safe.
All words above are used interchangeably. It's the amount of data that flows through your site, the amount of hits or how many people visit your site. If you're expecting a few hundred hits a day then 10, 15 of 25 GB of Transfer should be enough. If you're expecting hundreds of thousands of hits every month then you should get a hosting account with as much Bandwidth as possible.
So all these effects makes a good host.