persons and people

People is almost always used as the plural of person.  

One usage I can think of is in law enforcement. When a crime has been committed, they will search for persons unknown.  I understand this to mean that they don't know if it is one person or multiple people. 

There is a phrase that is more often used in the singular, but it could be used as a plural.  Before leaving the area, check on your person for ticks.  In this case, person is used to mean body.  When giving the direction to multiple people, you could say check on your persons.  This example isn't common, though. 

I agree with Patty.  People is generally the preferred plural of person.

Sometimes, persons would be used with numbers, particularly if there are other modifiers, but that depends.  Like Patty said, this usage is mostly used by law enforcement.

"Three unidentified persons robbed the bank."

I imagine persons can be used in a politically correct context, if you want to emphasize individuality of the persons or wish to remove gender from a sentence.  If "people" somehow feels rude to use, try "persons" and see if that sounds better.

In some places, it is best not to use either one.  In a work or military context, there are more suitable words like employees, personnel, staff, recruits, soldiers, etc.  Men can be used, but it may be considered sexist these days, whereas in the past, male pronouns were considered acceptable for situations where gender is not clear.

I always thought it was worded oddly, but I have seen several signs in various public buildings that read: Maximum Capacity 100 Persons. It doesnt always say 100, I only used that as an example.

It is correct, but it is uncomfortable (to me).

I suggest reading this helpful article on the subject.

http://www.dailywritingtips.com/people-versus-persons/