fMRI and BOLD - what do the pretty lights actually mean?

Functional magnetic resonance imaging (fMRI) relies on the blood oxygen level dependent signal (BOLD). fMRI is best known for pretty pictures of brains with lights on certain bits, which incidentally are often of little utility for medicine or science due to poor study design (see Uttal).
This paper goes from basics to advanced and is well worth a read if interested in the edge of current controversy.
Key points:

• BOLD signal arises from the relative overshoot in blood supply to increasing metabolic activity, leading to higher Oxy-haemoglobin, which is detected in venules.
• The most metabolically demanding activity in neurons is maintenance of resting potentials
• Glia are also metabolically active
• Local field potentials (LFPs) are probably the greatest cause of metabolic activity, and correlate strongly with BOLD in monkey experiments
• Neuronal action potentials (APs) have varying correlation with BOLD in monkey experiments, which may depend on when LFPs correlate with AP rates
• Different brain regions have different relationships between LFPs and AP rates; i.e hippocampus shows dissociation
• Dissociation can, theoretically, occur because of circuitry and vascular phenomena

Paper:

How and when the fMRI BOLD signal relates to underlying neural activity: The danger in dissociation

Arne Ekstrom, 2010