The term "Overdrive" came first... when people started... well... overdriving their tube amps in the 50's and 60's with current they got the tone that is now known as "overdrive" generally warm in nature with a good bite to it. They dude this by over "saturating" the tubes with tone so they pushed way past what they could put out in a clean tone... this gave it the harsh bite... Then when solid state amps came along they were no longer "overdriving" amps... they were simply cranking the gain and presence and volume and finding that on the outer reaches of the amps power band they got a really cool "distorted" tone. Distortion differs slightly in my opinion since only true tube amps can obtain true overdrive (the best tube overdrive pedals have real tubes in them also)... Distortion is harsher and by far more "distorted" you generally get better crunch from distorion. Those are the main differences in my mind... however there are a lot of people that use the terms interchangably with no knowledge of the true origins... and the fact that there is a difference... To me, overdrive is a tube amp term... and a modeling amp term... distortion is a solid state term.