When it comes to taking down a criminal and ensuring an arrest, gathering information is crucial for a number of reasons. The information will let you (and the police) know when and where a particular individual or group will be based on repeat behaviors. It will tell you whether or not they are violent or if they have been committing other crimes. Above all else, it gives the police the information they need to get people or groups off the street and where they belong.
There's an acronym we use in the Army, S.A.L.U.T.E. It stands for:
Size
Number of personnel
Number of vehicles
Number of equipment (weapons, cell phones, etc)
Activity
What are the people doing?
Who are they doing it with?
What are they doing it with?
Location
Where are the people located? (Street addresses, intersections, etc.)
If a crime was committed, where was the crime committed?
If the person or group is mobile, note where you intercepted or witnessed them and where they were going. (This means following at a distance)
Unit/Uniform
How tall is/are the person(s)?
What race?
Color hair?
Color eyes?
Color shirt? (Long sleeve, or short sleeve? What type?)
Color Pants?
Color hat?
Any accessories?
Time
What time did you come into contact with the person/group?
When did you observe specific actions?
When did they depart or arrive at places?
When did you break contact?
Equipment
Be specific if they were carrying something and describe it.
Be specific about vehicles that were being used. (Model, Make, Color, License plate number and state)