A sensor is an electronic component that translates physical states of the world into electronic signals that can then be read by a computer. They are INPUTS to a computer so it can know about the world. Like the keys on your keyboard, the optical tracking of your mouse. A light switch. A volume knob.
Overview of Sensors at S&&O
For getting started with sensors, here an illustrated overview with a selection of common sensors that we have at S&&O, with an attempt to organize/categorize them in some way. To find out more about each sensor, simply search it's name on Wikipedia or the Internet.
All the Sensors
(Of course) Wikipedia has a lovely long list of all the sensors that ever existed: https://en.wikipedia.org/wiki/List_of_sensors
O'Reilly Encyclopedia Series
The O'Reilly encyclopedia series on different components, the third volume in the series is particularly relevant for sensors http://shop.oreilly.com/product/0636920026167.do
There are many sensor kits you can buy where the raw component sensors are often mounted on breakout boards for easier, more robust access to their leads (legs) but also sometimes with extra components (resistors, capacitors, leds) on them to make the raw sensor components even easier to use.
Resistive Textile Sensors