|Left: Zigzag Goldenrod, Solidago flexicaulis. Right: Black raspberry, Rubus occidentalis.
Plant identification guides and keys often describe leaves as either simple or compound. This characteristic is helpful in pinpointing an ID because plants tend to be consistent in the type of leaf they have: simple, compound, or a unique combination of the two.
With a little practice, it's easy to tell the two apart. Simple leaves have continuous blades, like the one in the photo of Zigzag Goldenrod above. In contrast, compound leaves have blades divided into leaflets, like the five-parted leaf of Black Raspberry, also shown above.
The leaflets of compound leaves can be arranged in one of two patterns: Pinnate or palmate. Pinnately compound leaves have leaflets arranged along a central stalk, like pinnae on a feather. Palmately compound leaves look somewhat like a hand: The leaflets radiate from a common point, like fingers on a palm.
Pinnately compound leaves can be divided further, with primary leaflets divided again into secondary and sometimes tertiary leaflets. Leaves that are divided two, three or more times look fern-like.
For more information about simple and compound leaves, including examples of the types described above, see this video tutorial.