A problem at the colocation center, somewhere upstream from them, a power failure, schedueled maintenance, "acts of God" and the like generally also don't count as downtime.
So basically it's things like a hard disk failure, an OS crash, unschedueled maintenance, things the host is directly responsible for.
If you want true 9x% uptime and have it guaranteed in your contract with the host then you have to be prepared to pay for that as well.
If the host offers an independant link to a site that tracks uptime then that's useful, otherwise it's just another marketing trick.