RedBike said:
An audax is just that, a long group ride. (Although to call it a group ride is slightly miss-leading as it's possible to do the ride without seeing another rider all day. )
They're about the challange/ A way to experiance new locations by bike.
Sportives are probably slightly more suitable for a begineer as the routes are signed so no map reading and there's food /water stops provided around the route.
The difference between sportives and audaxes, as far as I can tell is the pace. Sportives tend to be ridden by folk who want to get around the course as fast as possible and audax riders want to eat as much cake as possible ;-)
I've not ridden a sportive but it is a race in all but name and is highly organised with waymarking and feed stations. They aren't always particularly cheap.
An audax ride is normally 50, 100, 200, 300, 400 or 600 km long. There's a high degree of self reliance i.e. no broom wagon, no waymarking - riders are given a route sheet and that's it. All food is bought/carried by the rider and events cost a fiver or less to enter especially at 200km or less.
100km audaxes are ridden at an average pace of between 10 and 15 km/hr.
They tend to be very sociable events though when I first started riding them I used to be lantern rouge and was on my own more often than not for the entire ride. Folk who think that a 100km audax is beyond them should think again - I'm a porky 22 stones and get around 100 and 200km rides with few problems. I even do the shorter Hebden Bridge based (50km) hilly rides and not finish last.
I'm riding my first 100km audax of 2009 from Thorne tomorrow. A nice flat re-introduction to audaxing after a five week layoff.