This obvious is very common, usually affects (Good) Students with a repeated microtrauma to the elbow or in manual labourers with obvious repeated strain on the elbow joint.
Typically affects people in the 30-60 year range as manual labourers are more likely to develop it than students (I wonder why).
Typically presents with a plum sized swelling on the olecranon, as you can see in the photo here, and is often red or discoloured but do not use this to rule out bursitis.