Can anyone give me a really simple explanation of how transistors work?
I'm looking to understand to what extend they activate and how to use
  them as a switch.


There are two basic types of transistor that are used as switches - bipolar (NPN, PNP) and field effect (MOSFET). 

A bipolar transistor can be thought of as a current amplifier with a diode between Base and Emitter. If the transistor has a current gain (HFE) of eg. 100 then it can switch a current up to 100 times greater than what is fed into the Base. But (due to the Base-Emitter diode) as Base voltage is increased the input current will rise very rapidly once it gets past 0.6V, so you need a resistor to limit Base current. Current gain varies with temperature and Collector-Emitter voltage, so to guarantee that it will be switched on fully under all conditions you should provide more Base current than the nominal HFE would indicate.

In contrast, a MOSFET is turned on by voltage between the Gate and Drain. The Gate draws no current, but does have some capacitance which must be charged/discharged. Again the response is nonlinear, but with this type of transistor you just need to ensure that the Gate gets enough voltage to turn on. The datasheet usually provides graphs showing the relationship between input voltage and output current, but the exact response is hard to manufacture accurately and varies widely between individual units. Therefore it is best to assume the worst case and give it plenty. MOSFETs are often described as 'Logic Level' (4.5V Gate drive) or eg. '2.5V' which specifies the voltage required to guarantee that they will turn on fully.      

Most electronics tutorials focus on the the physics of what goes on inside a transistor (electrons, holes, channel widths etc.) but you don't really need to know that stuff. Just read the device's datasheet and take note of how it responds to different stimuli. This information tells you what you really need to know - how a particular device is expected to work in an actual circuit. And examining the datasheet works with any device, even if you don't know what technology it uses internally.