A scene is one combat or social encounter, or the exploration of a reasonably large room or series of similar small rooms where an entire encounter might take place.
An act is several encounters (a typical map for an encounter site), a short overland journey, a quarter of the day (morning, midday, evening, night), or an extremely thorough exploration of a single room or area (taking 20).
The amount of time you spend exploring an area determines your ability to locate traps and secret doors, as well as other details. Taking 10 is the default for exploration, which takes 1 scene. Blundering through an area treats your Perception roll as 0 + any modifiers, but does not count as a scene for duration. Taking 20 to thoroughly inspect an area is one act for purposes of duration. You may always take a deliberate Perception Check (rolling 1d20 + mods) as a move action to glance at something in particular.