A sensor is a very tiny device that can basically do just one thing -- detect if something has touched it, for instance. A sensor in a mat at the side of a bed can detect that someone has stepped on it to get out of bed. A sensor under a bed post could detect if someone is tossing and turning. A sensor on a door could detect if the door had been opened. A sensor under a medicine bottle could detect that someone picked up and put down the bottle. A sensor in a light switch could detect motion in a room. A sensor next to a bathtub, toilet, or washing machine could detect if water has touched it.
A single sensor has some use. It could make a sound if it has been touched, for instance. However, to be really useful, a system of multiple sensors can send information to a central computer, where it can be aggregated into patterns that might describe what is going on in a room or a home. This computer could be in the same home or somewhere else, and could be set up so that someone like a distant caregiver can access it remotely.
It's not hard to understand the interest in sensors. They provide a way to remotely communicate what's going on in a less invasive way than cameras. However, the technology is new and there are many things that need to be ironed out.
Sensors are relatively cheap, and the cost is likely to decline as the market for them increases, so it is possible to use many sensors in a single home. The systems to aggregate and make sense of the information they produce are still very expensive.
Sensors may be powered by either a power cord or a battery. Battery powered sensors obviously can be placed anywhere, but someone will need to be sure the batteries get checked and replaced. Those that use electric power are subject to power outages.
Sensors are usually connected to each other and to a computer wirelessly, so it is not necessary to run wires. The raw data they send is not particularly meaningful in a vaccum, so it is probably less important than with cameras if someone were able to access the data sent over the wireless connection.
A system of dozens of sensors will generate a huge volume of data. Some systems allow the users to see all activity, but it's difficult to look at raw data and tell what activities are ordinary and which ones indicate problems. A key activity of researchers working on these systems is to create a way for the system to "learn" what is normal for the person being monitored. If this can be achieved, the system can be set up to notify caregivers anytime activities vary from their normal patterns. Doing this requires developing and perfecting complex mathmatical algorithms, an imperfect science.
A problem with sensor systems is that they work well when only one person is in the house, but develop problems when there is more than one person there. Guests, and even large dogs can interfere with the operation of the system. For the patterns to make sense, the system has to filter out activity that is not created by the target, and that is hard to do if anyone else is in the house.