I'm not very knowledgeable on databases.
Currently i'm doing a module at university and i'm creating an instant messenger. To keep track of all the users i have used a database and which socket they are connected to.

For my report I have to talk about the different database packages i have looked into (Access, MySql and Sql Server). I know that Access is regarded as the worst one because of concurrent connection and stuff and is only for small databases e.t.c but what are the KEY differences between the different ones?

Are they things like how many connections they can handle and if so does anyone know what type of ball park they are? 1000 to 10,000 connections ?
And backup possibilities?

Why is MySQL prefered over Access (Apart from the cost of course)?
Why is Access refered to as a simple database package?
And whats so good about SQL server?

If anyone can help with this i would be extreamly greatful!

Thanx in advance,