Why am I hearing other people talking?
When I am using my walkie-talkie, I hear other people conversations that I don't know.
These walkie-talkies, just like CB radios, operate on radio channels that are free-for-all, and there are only a limited number of channels available. Anyone listening on a given channel can hear anyone else (within a few miles range) speaking on that channel.
Good manners require that you don't break in on, or step on, other traffic, so don't start transmitting on a channel when you hear other people speaking, wait until there is a pause. If a channel is particularly busy, you might want to change channel. Of course your partner should also change to the same channel, so you announce briefly along the lines of "Agent 007, changing to channel X", and your partner should confirm with something like "Agent 700, copied, changing to channel X". ;)
These radios are on the FRS (Family Radio Service) band which is between 462.5625 and 467.7125mhz. These are no different than all the radios you find selling in pairs in all your big box stores. They all claim to have a 5-10 mile range.
NO, you cannot hear cops on them
NO, you cannot hear CB channel 19 with the truckers.
YES, anyone can get their hands on these.
YES, any business radio can be programmed to these including all the Chinese radios you find on ebay.
YES, as long as people aren't messing around on them, they are convenient to have to keep in touch in short ranges.