Keynote is the apple equivalent of power point which comes with Microsoft office but it forms part of iWork, the office package for Mac whish I think you can buy for about £80.
I have experience with using both powerpoint and Keynote and think that keynote is the way forward. Keynote makes making presentations easy and they do look a little different, which is good if you are, like me in a class where the class and marker has to sit through and watch 10 presentations, 9 in powerpoint and 1 in keynote, I know which one will stand out for good reasons.
To give presentations, my mac came with 2 things to plug it into a projector box, which are easy to use and pretty smart looking in the typical apple white.
Keynote has loads of options and one of my favourite is the slide templates which shows you multiple designs of slides and you can just choose which ever you need to use, like bullets and picture, title etc. The themes are carried out between the slides realy well, better than just giving them all a background on powerpoint, and this really helps the slides gel together. There is a wide range of preset themes so there should be something for everyone.
As with other Mac software such as pages, keynote makes lining things up wtih each other really easy and this can give your presentations a good polished look. I think this is much harder on powerpoint.
Keynote also has a lot of presenter options, for example you can see the slides which are next, you can add notes, you can record your slide show and you can time yourself too - all very useful if you don't want to mess up on the big day.
Keynote is frequently cited as the best app for presentations and I can really see why. It is user friendly, easy to learn, and easy to use to create professional looking, brilliant presentations.
Of the iWork suite I think it is the best and it makes the package worth buying!