If you are in business you definitely want your web site to be up all the time, or more realistically, at least not have much downtime. Web hosting companies know this and often offer what they call an "Uptime Guarantee." They will state that they guarantee that their servers will be up 99.x percent of the time.
An uptime guarantee sounds like a great feature. Sadly, it's mostly marketing hype -- even if a refund for downtime over the promised limit is included. First, there are a lot of exceptions to most uptime guarantees: power failures, Denial of Service Attacks, problems with the Network Providers the web host servers are connected to, etc. -- the very things that cause more major service outages. Second, no uptime guarantee covers the main reason business sites are down: problems with the software that you or your webmaster install to run your site (Wordpress, Drupal, your shopping cart, etc). Third, even a 99% uptime guarantee allows for over 3 hours of down time each billing period. Fourth, when you are paying $10 or less a month for hosting, even a refund for six hours of downtime is going to be less than ten cents.
While uptime guarantees aren't much as a warranty goes, they can be slightly useful in comparing web hosting providers. For example, a hosting provider who promises 99.5% uptime probably has more confidence in their systems than one who offers a 99% uptime guarantee. Then again, perhaps they just have a marketing department out of touch with reality.
One thing you can be sure of, however, any low-cost web hosting company offering 100% uptime is probably hiding a lot of exceptions in the fine print. 100% uptime just isn't realistic. Servers fail. Network connections fail. Power fails for longer than backup batteries and generator fuel will last.
In summary, while uptime guarantees may make you feel good, they really don't mean a lot. Therefore, don't put a lot of weight on uptime guarantees when you are selecting a web hosting company.