Indexed conference: A conference that is ISI or SCOPUS indexed, accepts only blind submissions, has rigorous referring (at least 3 reviews per paper) and low acceptance rate (less than or equal to 30%). In other words, in an indexed conference, if 100 papers are submitted, no more than 30 outstanding papers would be accepted. For examples, see
A sample list of SCOPUS indexed conferences is given in
http://www.vit.ac.in/files/sense/2009_c.pdf
Ranked conference: A conference is ranked based on its impact factor (higher impact factor means higher ranking), quality of published papers (ranked by post publication reviews and judgements by researchers), number of citations (high number of citations means high ranking).
Table 5 in the following paper gives a ranking of the top 10 conferences in 5 different categories (DB, Network, AI, Theory, IR):