DMR or digital mobile radio has been around for some time in several forms but hasn't really been popular with amateurs until recent years. We use relay stations called repeaters to increase the range of our equipment, just like phones do, and the the ability to connect these repeaters together via the internet has generated a lot of new interest.
Icom have a system called D-star which is well established and Yaesu have fusion which is new and growing slowly.
DMR is the system I have an interest in. It is based around equipment used by commercial operators.
Amateur DMR uses a system called TDMA or time division multiple access which allows two independent conversations to be on a single channel without conflict. This is achieved by two independent data streams taking turns to send, we call these streams "slots"
In practice this means one repeater can have local traffic on one slot with regional, national or world wide on the other.
The largest network of this type is known as Phoenix in the UK and dmr-marc or Brandmeister elsewhere. It is an excellent system when used correctly with it's only drawback being there are many options, known as talk groups, sharing one slot which can cause problems at times. It is therefore important to learn the correct protocol.