When you think of steampunk, you also think of clockwork and all of the little mechanical parts that go into it, which includes springs, wind up motors, gears, and cogs. All of these little ingenious parts work together in beautiful harmony to make things work.
Mainspring: usually a ribbon of metal that is the power source in gadgets utilizing clockwork. When you wind your watch, for example, you're twisting the mainspring tighter, which results in the mainspring storing potential energy.
Gear: a rotating wheel with teeth that, when used in conjunction with other gears, transmit the torque and make things move. That's why gears working together is called a transmission. There's all kinds of different gears: spurs (which are the most common), helical, and bevels. And gears can also be put together to form all kinds of neat components. For example, the sun and planet gear configuration is what powers steam trains.
Cog: just another name for the teeth on a gear. Many people confuse cog and gear - so now you know the difference!
Escapement: is how watches and clocks keep time. It's what gives a clock it's tick tock sounds, and as it moves (usually in a pendulum type of manner), it ocillating movement is what turns the gear in such perfect precision with each tick and tock.
What's cool about how clockwork is used in steampunk is how authors combine this ancient technology with more modern inventions, like automatons (or robots). A gear with it's spur-like teeth is a common symbol of steampunk. Steampunk just wouldn't be steampunk without clockwork.