1. What circumstances / conditions constitute a medical emergency ?
If someone's life is in danger then it would be a medical emergency. It is no different in the air or on the ground.
2. Who makes the decision to either divert to a place where suitable medical care can be given or continue with the flight?
Ultimately of course it is the Captain. However many airlines now have contracts with medical providers who they can contact whilst in flight. We can call up on HF and speak to a Dr/ Consultant who can advise us based on the medical information we give to them about the condition of the person on board. They will then advise us as to whether they think it is best to continue or divert, which then guides us the pilots.
3. Much as I know pilots have places they can divert to in the case of technical emergencies do they have similar locations for medical emergencies.
Technical or medical or any other emergency makes no difference. We always have en route and destination alternates available. If we had a choice of a few airports (perhaps whilst in Europe) then if there was time we may be guided to a specific one - never had it happen, but I guess that if there was a passenger with heart problems and we had a choice of landing at an airport near a heart hospital or one further away, and both were suitable airports, then we'd land at the most appropriate one. We would also be guided by company - sometimes an airport is better from a company point of view - for example if in Portugal and we had the choice of Faro or Lisbon we'd go to Faro because we fly there regularly and so handling etc. is all in place should the crew go out of hours etc. plus we would be familiar with the airport, whereas we don't fly to Lisbon.
4. Are there medical conditions passengers have that they should inform the CC of ?
If any passenger has a medical condition that requires a Doctors letter then this should be given to the cabin crew on boarding. It makes sense to tell the cabin crew anyway, because otherwise if you were taken ill during the flight and no-one knew you had a pre-existing condition then you may not get the appropriate treatment.