As Mick says, Outlook comes with Microsoft Office, which came already installed on my machine.
The 'Calander' function, which is what I am referring to, works just like a diary except you can modify, alter, and play around with entries as circumstances change.
You can also use it to store vast amounts of information. Lets say that you are due to start making Mrs Jones's wardrobe on June 10th. You might want to order the materials on June 3rd - so make an entry in Outlook's calander to remind you. You can add to this entry the shopping list of materials you are going to need. I do all this sometimes months in advance while the job is fresh in my mind after taking the initial order.
Then, on June 3rd, when you turn on your computer with your first cup of tea of the day, up pops a little screen reminding you, amongst everything else you've got on that day, to buy Mrs Jones's materials. Drill down and up comes the shopping list.
This is just one example of what it can do. I haven't even begun to use it to its full potential yet.
For a disorganised absent minded buffoon like me it is absolutely invaluable. I don't suppose anyone else is much different!
Cheers
Brad