Title is pretty much self-explanatory. When I disconnect (take out) my laptop's battery for a few hours, then put it back in and turn my laptop on, I haven't lost any information. How is this possible?!? Is time of the essence here, or are there like "on-chip" power supplies that keep the HD supplied with power for a long time?
There are two types of modern laptop hard drives, spinning plate and solid state. Both are non-volatile, and only require electricity to read or change the data. No electricity is required to keep the data stored.
In a traditional spinning disk platter, a very precise moving head permanently charges small precise parts of the platters to represent the individual bits of data. In a SSD, non-volatile flash chips are used.
If you're talking about a hard disk drive then you're misunderstanding how they work. A hard disk drive has spinning metallic platters. The information is stored magnetically and does not require electricity to remain "saved", only to be read or written.
If you're talking about a solid state drive, then you may wish to read the Wikipedia article on flash memory. Essentially it works the same was as the card in a digital camera or an MP3 player. Short electrical bursts are used to change bits of the memory, without requiring a constant power supply. If you're asking about an SSD drive, you may find the HowStuffWorks article on Flash memory or the eHow article on SSD drives interesting.
Compare those to RAM, (Random Access memory) which is what your computer uses while running and does require a constant power supply.