This is a pretty popular and fairly useful application. It has certain shortcomings, but I'll only gripe briefly about them. First off, the way iphoto works is very similar to itunes (or many mac applications for that matter). When you import images into iphoto it copies them to it's own folder and then organizes them in this folder. There's many ways to organize your photos. The main two are rolls, albums, and keywords.

Rolls are more of a temporary means to organize. The default setting is to have a shortcut to the "last roll" in the upper left of the window. That is to say, the last group of images you imported.

Albums are fairly self explanatory, as they are simply a collection of photos. Once you have imported your images you can select them all and drag them to the left area and it'll create a new album with those images.
Lastly are keywords, which are words you assign to certain photos that enable you to find them later. Not as useful as they could be, because you have to CREATE the keywords before you can assign them to a photo.

You have many options in regards to what you want to do with them once you've imported them into iphoto. You can make slideshows, cards, calendars, books etc. You can create all of these and the cards, calendars and books can be printed and shipped to you by apple. My one biggest complaint is you inability to add any kind of text to slideshows. It's very aggravating. Another I've learned while browsing some blogs is that iPhoto also loves to hog hard disk space. The thing is that whenever you delete a photo, it doesn't actually get deleted. Instead it just moves to iPhoto's trash, which is really just another place of storage. To free up space, go into this "trash" and delete the picture once more to truly remove it.

That's all.