Conference vs Congress vs Symposium vs Meeting
There are certain (informal) nuances I believe:
Symposium - Prestigious conferences, generally leading venues in their respective fields. Example: Symposium on Discrete Algorithms, European Test Symposium, Symposium on Foundations of Computer Science (FOCS) etc
Conference - Regular venues for publications, may range from established venues to the archaic. I understand the bulk of publications of most researchers are in one conference or other, as symposiums tend to have a very low acceptance rate.
Meeting - I'm not so sure there are many of these, but I understand that it is more of a forum for interaction/surveys/posters than for publication of full papers. (I based my answer on the description for SIAM Annual Meeting 2012, which describes itself as providing "a broad view of the state of the art in applied mathematics, computational science, and their applications through invited presentation, prize lectures, minisymposia, and contributed papers and posters".)
Congress - This would typically be held once a year per discipline, highlighting the achievements, notable results in that field. These are typically attended by leaders in that field, and feature a series of invited talks (for example, look at Mathematical Congress of the Americas 2013).
Nothing. They're synonyms. See also "Workshop".
"Conference" or "meeting" are catch-all terms that can refer to any scientific gathering. However, I'd argue that a "Symposium" tends to be smaller than the others, and more narrowly focused. "Conferences," "Congresses," and "Meetings" can all be huge affairs, but it's hard to think of many "Symposium" with a similar size. A "workshop" is also an event of somewhat smaller size than a "congress" (which generally implies a huge attending audience).