I used to know this, but I looked it up to make sure, and here's my understanding:
-It takes 16 senators to call for a vote to end debate.
-After calling to end debate, the cloture vote must be held within 30 hours of 16 senators moving for cloture. During these 30 hours, lots of things can happen including roll call votes and quorum calls, but individual Senators are permitted only 1 hour each to speak.
-Once the 30 hours are up, a 3/5 majority is needed to invoke cloture. Failing to vote for cloture is equivalent to vote against it, ie, you need 60 votes to end it, regardless of how many votes show up for the other side (unless there are vacancies in the Senate).
So functionally, this means that if there are not 60 votes for cloture, any business in the Senate can be delayed indefinitely. Since it takes a full day to hold a vote on cloture (ie the 30 hours), it is pretty costly to actually go ahead with a cloture vote that is bound to fail if there are known to be fewer than 60 votes. In practice, then, a credible threat to filibuster imposes a 60-vote supermajority requirement on anything 41 senators want.