Column Order in a Composite Index

One of our readers asked 3 questions about composite indexes which we thought we would share with all of you:

Q1) For a composite index, is the order of the columns important and if so, can you please explain it with examples?

Q2) If I have all the columns of the composite index in the filter condition (WHERE clause), does the order in which they are specified matters?

Q3) When designing a composite index, assuming that all the columns of that index are going to be used in the filter condition, does it matter whether the most selective column i.e. the column with most distinct values is the first column in that index?