For those that need a refresher on airspace classifications, I have provided the community with an easy to read photo; brought to you in part by the FAA.
However, for those that are starting off, with your first drone sitting in front of you... This diagram is a must of knowledge to have, when operating any size UAS (Unmanned Aerial System). Stay below 400' AGL (Above Ground Level). If uncertain, of the airspace that you are in, you can download apps such as:
Air Aware
B4UFLY
FAA Class Airspace Viewer
Airspace Maps on the App Store
FlySkyHy Soaring App