Steampunk aesthetics come from a sub-genre of science fiction of the same name that incorporates technology and designs inspired by 19th-century fashion and industrial machinery (including steam-powered machinery, hence the name). Steampunk fiction often explores the anachronistic idea of what would have happened if society built upon steam as a primary energy source and maintained the Victorian visual style.
Steampunk engineering often includes visible gears and screws, as well as analog clocks, steam-powered machines (including vehicles such as airships that often carry flying wooden ships), rotating propellers and dials. Those are always made of metals, wood, or even glass, but never plastics or other modern materials.
Steampunk fashion often includes Victorian-style clothing, such as suits, waistcoats, top hats and long dresses, as well as inventive pieces such as goggles and short (but still old-fashioned) skirts.
A lot of Steampunk is, much like its futuristic counterpart Cyberpunk, rooted in literary traditions, especially among the works of Jules Verne and H.G. Wells. While it doesn't exactly share the nihilistic worldview often seen in Cyberpunk (opting instead for a more romantic worldview), it does share the rebellious spirit Cyberpunk has instilled in its aesthetic DNA, often used as a critique of the Victorian social etiquette that was popular in the time period by deconstructing or outright challenging the notions head-on.