by 
The Curious Bag
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A spore and a pollen grain are both microscopically small reproductive mechanisms for living organisms, but there are many differences between them that can be used to classify them as spores or pollen grains.
Production
A spore is produced for reproduction by bacteria, fungi, and non-flowering plants. A pollen grain is produced for reproduction exclusively by flowering and cone-bearing plants.
Germination
A spore can germinate asexually without the need to fuse with another spore to create a new organism. A pollen grain cannot germinate and create a new organism without first being joined to the pistil of a compatible plant species.
Dispersal
A spore is forcibly ejected from the reproductive mechanism of the organism that created it. Grains of pollen must be carried by wind, insects, or animals to other plants of the same species that need fertilization.
Durability
A spore is durable in unfavorable conditions and can germinate after years of dormancy. A pollen grain will lose its ability to germinate if specific favorable conditions are not met.
Size
A spore has a smaller maximum size than a pollen grain. The largest spores can reach a size of 20 microns, while the largest grains of pollen can reach a size of over 200 microns.
Source:
Pollen: Collecting, Observing, and Classifying; Greg Bisbee; 1997
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