Some good comments... I'd like to expound upon a few if I may...
Simply, the lower the frequency, the further it has the possibility of being heard. Now there are numerous factors that will play into this. Output transmitting power, antenna (height and physical length), atmospherics (especially on the VHF-LO [usually the 30-50 MHz band]) and other things.
For one thing, free space loss is directly related to frequency, so all else being equal, lower frequencies have less loss. The difference is significant between, say, low band, and 800.
One reason lower frequencies tend to be able to be heard further is their 'wavelength' is more easily reflected off the ionosphere (called skip. If you ever listened to AM radio in its heyday, it was not uncommon to hear distant stations [there is a thread on that topic in the general forum] fairly clearly at night).
Absolutely true. HOWEVER, there are reflective enhancements that primarily benefit higher frequencies. Meteor scatter comes to mind. Tropospheric ducting is more prevalent at higher frequencies... to a point.
As frequencies climb up, they become more and more line of sight. Now no radio wave can bend with the curve of the earth, but the skip capabilities of the lower frequencies do allow signals to be heard over the horizon.
Well, truth be told, because of the gradient in atmospheric density with altitude, radio waves DO follow a curved path. In radio engineering, this is actually taken into account, and is referred to as 4/3 earth radius. If the earth were one third larger than it is, radio waves WOULD just go around and around. The radio horizon is about 1/3 further than the optical horizon.
There is no one ideal frequency range. Depending on what you are trying to do can have a lot to do with what range you transmit on. Again, there are some restrictions due to law and international treaty and as radio spectrum becomes more and more crowded, these ranges or options change. A good example is the 'rebanding' of the 800 MHz range in the US. Read the Wiki here for info on that topic (not to mention the numerous forum entries on the topic).
Range can vary so wildly that I usually try to avoid even discussing it. It's a complex combination of factors that include transmitter power, frequency, the bandwidth of the modulation, and receiver noise figure (which is a whole book length topic unto itself). Frequency is just a part of it.
There are two ways a radio signal can be transmitted (I'll easily stand corrected on this one guys). Either Amplitude Modulation (AM) or Frequency Modulation (FM). Most public service is FM and because it is a bit more stable (less prone to interference) than AM, digital signals work very well over FM. The main areas of AM transmission around today (that I can think of) are AM radio (500 - 1600 KHz), SW radio, aircraft (military and civilian) bands and (though technically it is in the HF area shared with SW) good old CB.
You can modulate the frequency, amplitude, and the phase of a signal. Every other modulation scheme is a variation, or combination of these three things. Even digital modes.
Each band has its own pros and cons. One of the cons to a lot of the 800 or higher systems is they don't work real well inside a building or in very rugged terrain without a nearby repeater. The construction tends to block the signals. Other bands may have problems in rural areas where trees or mountains can block signals.
One of the most insidious things about this is that 800 MHz is one of the worst pieces of spectrum to use for public safety communications, based on how it behaves. But it's what was available. Trees are a detriment at higher frequencies, generally, and certain species of pine trees are particularly bad because of the size of the needles in relation to the signal.
One of the main reasons (IMHO) for the increased use of higher frequencies has been the crowding of the spectrum. Everyone wants a radio system and the lower spectrum could not support the expansion. Where in the 150 MHz area you might be able to reuse a frequency every couple of hundred miles to avoid interfering with other systems on the frequency, with an 800 MHz system it may drop down to less than 100.
Yep yep! That's exactly what's pushing things higher. There's no upper cap, although I doubt we'll see gamma ray wavelengths used for communications.
All of the above that LEH said is true, but there are some interesting exceptions. For example, I'm active on amateur microwaves, and I have a very high performance 10 GHz ham station that will out talk and out perform VHF and UHF, and performs more like HF when the band is out. 500 mile contacts are a piece of cake, when VHF just doesn't make it.