I have been trying to understand what is exactly a sieve and why sieves are useful. I have read Wikipedia articles about sieve theory but they don't provide a definition of what is a sieve or why they are useful.

Can someone explain what is a sieve in general terms and what properties of sieves make them so useful in attacking number theoretic problems? Are there general principles that one can use as a guideline to see if sieves are likely to be useful in attacking a problem?

I know this question might be a bit elementary for number theorists, but I wasn't able to find good and concise information at the level of a general mathematician. (The Wikipedia article links to Ben Green's notes about sieve theory but the link is not working anymore. I Googled a bit however I couldn't find a concise high-level but rigorous exposition, notes like this and this dive quickly into examples and applications without giving any general definition and discussion of sieves.)

Opera de Cribro, by Friedlander and Iwaniec. – Andrés Caicedo Mar 17 '14 at 5:25