There are several possibilities. The most common cause is a small benign nodule called a Heberden's node which forms along with osteoarthritis at the tip of the finger. A similar nodule called Bouchard's node forms over the PIP joint in the middle of the finger.
These nodules are an early sign of arthritis and the arthritis may not be painful at this stage.
Other causes may include ganglion cysts or "knuckle pads" which are associated with a problem called Dupuytren's disease.