Tempo Engines

The tempo engine runs in many weenie decks, and in control decks. Their extremes in spectrum bring them closer together when thinking about tempo and how it works as an "engine."
This broad definition covers any deck that isn't a “combo” deck (i.e. one that needs certain pieces in place to win). If your deck gets some sort of advantage and wins, then it's tempo. If it needs such-and-such in play, then it's combo.
Let's take two of the best aggro decks in the format, Gruul and Ghost Husk, and how they fit into the mold.
Both of these decks rely on explosive openings, one more so than the other. That is, Ghost Husk has the Dark Confidant engine, while Gruul has reusable boosts (Moldervine Cloak) and burn (extreme swings in tempo).
When you play decks of this sort you have to “feel out” your opponents' answers, and try to get in as much damage as possible while you dart in and out of their blows. At some point even instincts can take over, as you “feel” their Wrath of God, and plan accordingly.
On the other end of the scale are control decks. Let's take Izzetron and Firemane Angel Control for example. Both of these decks try to create unfair situations - Izzetron pops a Wildfire, Firemane gives you the Zur's Weirding lock.
The difference is that this tempo engine is a slow burn - for example, in cars in extreme weather conditions, oil can solidify to sludge, and as a result the engine needs a good, long time to warm up before it can perform. Control decks are much the same - they take extreme heat (Burn) and pressure (Creatures / Enchantments), and need time to get resources online. These decks always need some sort of lifegain or card advantage to make up for their lack of threats or removal.
The key to any Tempo Engine is recognizing how it should win, and what it needs in play to win. If it's about building pressure, then what kind of pressure? Answers such as counterspells, or threats such as Scab-Clan Mauler? If it's about finishers, that means reset switches like Wrath of God and game-enders like Char.