Disk space is your allocated amount of space on the hosting server. In case of shared hosting you have a set amount, in case of dedicated hosting you are limited by the size of your installed hard drives. Web pages are usually very small, so you can store a lot of web pages in a small amount of disk space. You should not worry about space unless you have an e-commerce site with a large database of products and images or if your site offers files for downloads or allows the users to upload files. In this case, you have to plan ahead to accommodate these files, so you need to look for hosting packages with appropriate disk space.
When people are visiting your website, they are in fact downloading the web pages and images via their web browsers, on their computers. When you are copying or updating files on your website account, you are uploading data on the server. Bandwidth measures all this activity: download and upload. So, bandwidth is somehow related to disk space, because it measures the number of bytes transferred by a visitor when he accesses the site's files. A site that has many pages and images will "consume" more bandwidth for the same amount of visitors (assuming the visitors will browse more than the first page) than a website that has less pages or the pages are mainly text. So the rule here is "the more the better".
Bandwidth is usually charged on a per month basis and it's calculated in MB or GB. There are web hosts that offer unlimited bandwidth. Bandwidth and disk space cost money, so it's not possible or at least not economically feasible to offer one of them or both in unlimited amounts. So, why are there offers with unlimited bandwidth? The hosting company is counting on its traffic statistics; around 90% of the websites don't use a great deal of bandwidth (less than 2-3 GB per month) so, on average, the "low bandwidth" sites will compensates for a few "high bandwidth" web sites. We're not saying to avoid hosting plans that offer unlimited bandwidth, maybe the hosting company pro vides good service, but make sure you read carefully the agreements or the terms of service, as there will probably be a note about what unlimited bandwidth really means.