A radio system can be as simple as 2 radios, capable of transmitting and receiving, which is used by many small towns for their snow plows to talk to one another. Or it can be as complicated as over 1000 radios capable of selectively talking to certain groups of users such as police, fire, EMS, etc. In the former case, there is no central base station that "dispatches" the plows. In the later, there is usually one central 911 center capable of communicating to all groups, either at once or selectively.
There are two basic types of large systems: Dedicated and Trunked. A dedicated system can have 20 or more frequencies(channels) where only one conversation can be held on each channel. For example, take the town that has 2 channels. If there is a fire and a police emergency, those 2 channels will be tied up and the EMS units will be unable to communicate during this dual use emergency.
Trunking is a way of sharing those 2 channels so that all 3 will be able to use them. A smart interface will constantly seek out the channel that is momentarily not in use, and allow any unit to use it. That is about as simple as it gets. However, trunking is usually used for 6-20 channels and 100's of users, but the principle is the same.
Now, you can get all bogged down with the type of radio signal(AM or FM), or the frequency(Low band, high band, UHF, etc), but any radio system can use AM(not used much except aviation) or FM, and any frequency. There are trade-offs with each, but you wanted a simple explanation.
Then there are repeaters. They are simply radios placed at key, and usually high locations, that can receive mobiles and repeat the communication from this higher location with higher power that effectively allows a mobile that is normally restricted to only 5-10 miles to be heard by all units for as many 30 or more miles. This is often done in hilly areas where there are dead spots.
