Most simple circuits that are using a current driven component will use an NPN transistor. This simple circuit uses an PNP so we can explain the differences between the two.
Reviewing the above schematic symbols you should note that to turn on a NPN type transistor you will need to use positive current on the base lead.
To turn on PNP transistors you will need to use negative current or at ground level on the base lead.
NPN should have the load on Collector side which is controlled by positive current flowing to the base which then switches the Emitter to ground allowing current to flow from Collector to Emitter (calling this 'sourcing' the load).
For PNP type transistors, the Emitter is on the positive side of the supply voltage and the load is on the Collector side but sinking it to ground via negative current going to the base (listed as V-, Negative Voltage or at Ground).
(See our "Transistor Basics" article for a simpler but more details discussion on Transistors, and also for a different perspective explained another way, see our eewiki article titled How to Drive a Stepper Motor that may help further define how transistors function)
Creating the circuit
Regarding the Night Light circuit below - we wanted to make use of a PNP transistor for circuit explanation (most discussions on the web are around NPN types)
and we wanted to create an easy, interesting circuit to help young students build a useful circuit they can experiment with (... as part of our Digi-Key local STEM outreach program).
By using a CDS photo-conductive photocell, we can create a night light (or a low light level indicator) by using just a couple resistors, a transistor (in this case, a PNP- 2N3906FS) and an LED in the schematic drawing below.
220KQBK-ND - 220,000 Ohm quarter watt leaded resistor
5.6KQBK-ND - 5600 Ohm quarter watt leaded resistor
N145-ND - 9 Volt battery
1597-1268-ND - Small breadboard - aprox. 1.75 inches x 1.25 inches or slightly larger
36-84-4-ND - 9V battery snap
377-2093-ND - Jumper wires
Probably the best book on Electronics that I've ordered and tends to be a great standard for referencing funcitons of electronic components is:
Book "The Art of Electronics" by Paul Horowitz and Winfield Hill - 2nd or 3rd edition
Another book worth purchasing that tends to be more 'Maker Friendly' is the Electronics CookBook by Simon Monk