There's a wide range of devices. For small apartments or single room protection, there are clock radio sized motion sensing alarms. They offer portability and convenience, but are silenced with a $1 hammer. Next up, there are battery powered sensors that use RF to a central alarm unit, so that doors, windows and motion sensors can be utilized without new wiring. Remote systems allow you to place the alarm unit wherever is best. The downside is keeping track of the batteries in each sending unit and the size of the sensors.

Hardwired units are typically part of monitored systems (you pay a monthly fee) but they can be installed to be pretty discrete.