"Rajdhani" means capital in Hindi. So, Rajdhani Express connects Indian state capitals to the National Capital New Delhi. They will always originate from Delhi area. These are all fully air-conditioned trains enjoying the highest priority on the run. They can be trusted a lot more in case of punctuality than other trains. They halt only at important cities on the route.
On the other hand, "Duronto" is a Bengali word meaning quick. So, Duronto Express offers the fastest connection between any 2 cities between which it runs. They can originate from any city in the country and are not specific to Delhi region itself. These come in two versions-1 is fully air-conditioned and another is a mix of AC and non-AC...
more... coaches. They come 2nd in the priority list. When first started, these were conceptualised as trains which will have only end to end passengers just like non-stop flights ,taking halts only for operational reasons like crew change, watering, food loading etc. But it was soon found out that they some of them were loss making due to their non-stop nature. So, now they are provided with commercial halts. So, in other words, a Duronto is a Rajdhani for connecting various parts of the country to each other. Hope this helps. Do contact if you have any further queries.....Regards...Ankur