Redstone and electricity have much in common. They both have what's called resistance. That is, they only travel a certain distance before the signal degrades to the point where it's unusable, and they take a certain amount of time to get there. With electricity, that time is nearly instantaneous because electricity is extremely fast. Redstone, on the other hand, is pretty slow.
When you start involving repeaters and redstone torches, that speed slows down to what can seem like a crawl.
Here's how it works:
Now let's see how to build one!
Step 1 Make a V
Start with three blocks in a V-shape. I've used diamond for this build because I'm swanky like that, but you can use any blocks you like.
Step 2 Add a line of blocks behind them
Above and behind your V, add a line of blocks.
This is a somewhat confusing, complicated build, so bear with me!
Step 3 Pistooooon poweeeeeer!
Add a sticky piston in the corner on the right side. It's hard to describe, so just look at the picture.
Step 4 Another block
Put a block directly underneath the piston.
Step 5 More pistons, for fun and profit
Now we're adding two more sticky pistons, facing the same direction, above and in front of the piston you've already placed. See the picture below:
Your build should only be two blocks wide, so make sure you don't add your pistons off to the side.
Step 6 Glowstone
Put a glowstone block directly above the back piston.
Step 7 More blocks!
Looking dead-on at the pistons, add a block in front of each of them, and then one underneath the left block.
Step 8 *sigh* More blocks
Now put another block directly in front of the bottom piston.
Step 9 A lonely block
This block is lonely. It sits by itself.
Step 10 Give the lonely block some far-away friends
Add some blocks above and in front of the lonely block. They aren't really his friends... They look down on him.
Step 11 No more blocks! Time for redstone!
Run some wire across the V you started with.
Step 12 Do I need to repeat myself?
Now run wire across the back, and put a repeater (on the last delay setting) pointing at the glowstone block.
Step 13 Torches sans pitchforks
Attached a redstone torch to the block underneath the bottom piston.
Step 14 More torches... Morches
Now attach a redstone torch to the block below and in front of the left piston, if you're looking at this device head-on.
Step 15 More wire... Mire
Now add redstone to the Lonely Block (TM) and his two friends who look down on him. This bit of wire is your output, and you can connect other things to here.
Step 16 Add a switch!
Now add a switch anywhere on your starting V, and you're good to go! Voilà, instant power!
Now go make redstone devices... and be quick about it!