Is there any tiers or levels for conferences? and if yes on which basis people and authors can decide that a particular conference is of Tier 1 and another one is Tier 2 ?
Depending on your discipline, you may be able to find an established ranking of conferences. Computer science-related disciplines heavily gear their publishing towards conference proceedings and, as such, there are resources, depending on the sub-discipline, that can give a hierarchical ranking of 'top conferences' (perhaps, depending on your discipline, a simple Google search could resolve).
As has already been mentioned, conference prestige is often associated with levels of geography, "international" being the highest level. This typically means anyone can submit and so, given the larger audience, there is usually correspondingly larger degree of prestige associated.
Also, it may worth noting that conference impact factor statistics has been an emerging trend as of late. This may also be used, where available, to assess conference prestige.