In the navigational world the letters AIS stand for 'Automatic Identification System'. This is radio device which, since 2002, all passenger vessels and most of those of more than 300GT (300 gross tons) are obliged to carry. Smaller vessels can do so voluntarily. I have to tell you that, although not to be 100% relied upon nor replacing human watch keepers, I believe this to be the most reassuring boon to collision-wary sailors ever devised.
At intervals of not more than 10 seconds, each ship must transmit information including its MMSI (Maritime Mobile Service Identity – a 9 digit unique number associated with its VHF installation). Basically this is like a telephone number  that allows you to use DSC (Digital Selective Calling) to talk to any similarly equipped vessel directly. Also transmitted will be the vessel's call sign, name, bearing, distance off, speed over the ground and course over the ground. Hugely useful is CPA (Closest Point of Approach) which raises an audible alarm if it thinks you may pass too close. The positions of each vessel within range (up to about 20 miles) was for us displayed on both the chartplotter and our VHF radio.
Other information may also be revealed:

• Dimensions to the nearest metre
• Draught in metres
• Cargo type
• Destination
• ETA (estimated time of arrival)

Quite apart from the safety aspect, AIS brings some interesting entertainment value. To know that a ship, perhaps not yet even visible, will pass you perfectly safely at four nautical miles distance and is carrying bicycles bound for Bangkok is to flesh out what would otherwise be a an anonymous encounter. When Shindig crossed to the Caribbean in 2015 we only had visual sightings of a couple of ships far offshore (more when we closed the shore) but, thanks to AIS, we were aware of the presence of a few others below the horizon.

One sobering point though. Our AIS set has a receiver facility only. There are parts of the world where a sailboat's characteristics (erratic course, relatively slow speeds) would identify it as such, possibly attracting the attention of those who might see you as easy prey. Granted, the transmission facility can be switched off but, since we couldn't imagine Shindig being much of a threat to other vessels, we decided against burdening the airwaves with superfluous signals.