Radio ‘Channels’ usually refers to the Frequency that the radio transmits on. Most radios can use more than one channel. All radios need to be on the same channel to talk to one another.
Radio spectrum (the array of available channels) is under huge demand so radio channels are allocated to users by Ofcom.
In an ideal world a radio channel would be allocated to a single user for their exclusive use but due to demand this is not usually feasible. Radio channels have to be shared with other users. This is not a problem if the other users are out of range (geographical separation). It is mostly not a problem if other users are within range because the Frequency Modulated (FM) technology commonly used for business communications has the virtue of only responding to the strongest signal and it is almost always only radios in your group that are the closest.
Radio channels allocated by Ofcom are licenced and there is a small fee for using them. Many users only need very short range radio - same building conversations and for this purpose there are a number of channels allocated for use licence-free.
All business radio have other features to prevent you having to hear other users on your channel. The commonest method used on analogue radios is CTCSS. Digital radios use more complex technologies including encryption.
If you have a large fleet of radios then the situation may arise where you don't want to hear certain radios rather than all the radios in your own fleet. For this situation Selective Calling is used.
Analogue radios will only allow one conversation per channel. Digital radios can usually handle two conversations per radio channels.