D flip-flops are much like ice cream, they come in many different flavors. And regardless of the flavor you have, it's still a D flip-flop.
We are going to look at a simply D flip-flop first. The other flavors just build off the basic one.
_______|_______ Truth Table
| | ----------------------
D | | Q D CLK Q \ENABLE
----| |---- ----------------------
| | H ^ H L
| | L ^ L L
| | X L Q L
| 74LS374 | X X L H
CLK | | ----------------------
----| | X = Don't care
| | ^ = LOW to HIGH transition
|_______________| H - HIGH (voltage)
L = LOW (ground)
Q = Output, CLK = Clock
Three inputs and one output. Can it get any easier? Well, one input and no outputs but I don't know what the heck that would be.
Here's how it works. Q is going to output whatever value is on D right as the clock goes high. If the clock signal is already high when D changes the output does not change. Only when the clock signal is rising is there a change. This is what is referred to as a postive edge triggered D flip-flop.
And the truth table says all this and more, just not in so many words. It should also be pointed out that when the CLK is low, Q stays Q. Meaning, it keeps whatever value it has when CLK went from low to high.
But we have neglected the \ENABLE signal so far. When the \ENABLE signal is LOW, the D flip-flop works just as we described above. But when the \ENABLE signal goes high, Q will always be low, regardless of whatever D and CLK are doing.
May you have noticed how the \ENABLE signal works in the opposite way that one might expect. It would seem more intuitive for the D flip-flop to be enabled when it gets a HIGH signal instead of LOW. However this is not the case since the \ENABLE signal uses negative logic. (Normally you would draw a line over ENABLE, but here we will use a \ to depict negative logic.)
Think of it this way, when \ENABLE is HIGH the negative of it would be LOW. And if \ENABLE is LOW, the negative of that is HIGH. So a LOW on the \ENABLE signal really means a HIGH in a roundabout sortof way and the chip gets enabled.
There are other flavors of D flip-flops you can use, like those mentioned above. Some have SET and RESET signals. Others don't use a clock signal to set the value of Q but just a normal signal. (That truth table is subtly different.) Many have two outputs, Q and \Q. Where the \Q is always the opposite Q. (Remember that bit about negative logic?) But when it comes down to it, it's all just a way to take an input signal and hold it the output for a certain amount of time.
What can you do with D flip-flops? I thought you'd never ask. They can be used in shift registers, binary counters, frequency dividers, FIFOs and anything thing else you want to cook up.
This write up is based off the DM74LS273 datasheet.