If I could only afford to buy one light meter it would be a spot meter.
An incident meter reads light falling on the subject. Hold this meter in front of the subject and point the spherical dome toward the camera. A reflective or spot meter reads the light reflected off of the subject and requires an understanding of the Zone System to determine an F Stop. Simply put, a spot meter wants to put everything it reads at ZONE 5 or 18% neutral grey. Any other value requires an exposure compensation to achieve proper exposure. Here’s a very common example: Spot reading faces. Generally speaking, Caucasian faces are Zone 6 or one stop brighter than Zone 5. Since the spot meter sees everything at Zone 5, your reading of that typical face will give you an F stop that is one stop under proper exposure. So after a face reading, always open up one stop to achieve full exposure. For clouds – open up 2 or more stops, for snow on a sunny day – open up 3 or more stops. For Zones 0-4 you must stop down to achieve the proper value as the spot meter brings those darker values UP to Zone 5. DSLR cameras are equipped with reflective metering systems.
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