Does a low acceptance rate imply quality? I am presenting a paper in a conference with a 20% acceptance rate. However the conference is just 3 years old and does not figure in the tiered rating system for conferences in my field Computer Science (that some universities use) . Recently I found out that another reputed international conference (that figures in the conference rankings) has almost double the acceptance rate. Did I make the right choice. How should I evaluate conferences in the future??
Low acceptance rate does not necessarily imply quality, but it is correlated.  Crappy conferences often have high acceptance rates because they don't care.  Broad-community high quality conferences often have low acceptance rates because lots of people are trying to get their material into the same conference.  

These are just correlations, however, and not causal in nature.  For example, some very good conference have high acceptance rates because they serve a highly specific community; thus they tend to draw high-quality submissions from within that community and few submissions from outside.

Complementarily, I know of at least one conference that games its acceptance rate by artificially regulating the number of papers accepted.  They decide in advance how selective the conference will be, and set the cutoff not in terms of quality but in terms of their judgement of "good conference" acceptance rate.

Finally, having too high a selectivity may be bad for a conference.  A colleague of mine who is an insider at one of the extremely selective CS conference told me that they were worried that their acceptance rate was too low, and that therefore they weren't getting very interesting work any more: a single dubious reviewer was enough to sink a paper, so only conservative and incremental work was making it in.

Thus, acceptance rate is not a very good metric.  It's much better to look at who the people are who go to the conference, what they've been publishing there, and whether you want to be associated with those people and that type of work.