Particles (About.com)

Particles are probably one of the most difficult and confusing aspects of Japanese sentences. A particle (joshi) is a word that shows the relationship of a word, a phrase, or a clause to the rest of the sentence. Some particles have English equivalents. Others have functions similar to English prepositions, but since they always follow the word or words they mark, they are post-positions. There are also particles that have a peculiar usage which is not found in English. Most particles are multi-functional.