Most of the time,
the is just a
definite article, required by the rules of
English grammar. When given special emphasis in speech, however, it becomes a sort of
superlative.
Compare these sentences:
"Microsoft is a source of great suffering in the world."
"Microsoft is the source of great suffering in the world."
Grammatically, you're saying that it is the only source of great suffering in the world, but in speech, the word the can be given special emphasis. It's not just "thuh source", it's "THEE source".