Data loggers are electronic devices that record data over a period of time or changes in location using inbuilt instrument sensors or external sensors. In most cases they use a digital processor to perform their functions but some do not.
A data logger is usually small and portable. It is powered by a battery that runs a microprocessor (or a mechanical device), the sensors and memory for data storage. Some can interface with computers which run applications that are used to view and analyse collected data. Others come with a small LCD display and keypad and can operate as entirely stand alone units.
It is common for the device to be programmable but they usually have few or a limited number of parameters that can be changed. They usually operate in 24 hour ranges and once activated can be left unattended for the period to be measured.