A hosting account allows your website to be accessed by anyone on the Internet. Specialist companies provide space on a 'Server' (a physical / virtual computer dedicated to running one or more services) to host your website and also provide Internet connectivity to allow public access to the website.
These servers are located in secure environmentally controlled buildings with backup power supplies and Internet connections, often referred to as Data Centres. Hosting your website in a dedicated Data Centre with redundant services means that if a particular service fails, a backup service immediately comes into play, thereby providing uninterrupted access to your website.
The Availability of your website is measured by the percentage of a year in which the website is publicly accessible and reachable via the Internet.
The Uptime of your website is measured by the percentage of a year in which your website is up and running and online. This is different to availability, as it doesn't take into account if the website is reachable via the Internet which would be the case if there was a problem with the hosting companies Internet connection.
Modern Tier 4 Data Centres are able to quote very high availablity figures, e.g. 99.9999%. This equates to a downtime of around 31 seconds in a year.
The hosting company will also be responsible for backing up your website.
All of the above and many other factors are all used to define the Service Level Agreement.
There are many hosting companies to choose from, each offering similar services, but if any or all of this is too daunting for you, we can take care of everything on your behalf.