If you're new to Aperture it can be a little confusing. You may be used to organising files in folders on your Hard Drive. This is a perfectly good way to organise files but limited for organising Photos.
Remember that file is just a container - a box - for your Photograph. Think of it this way: In my iTunes Library I have a file called 'Let_it_Be_The_Beatles.mp3'. So what is that, exactly? It's not the song. The Beatles never wrote an mp3. They wrote a tune and lyrics. They recorded it and a copy of that recording is stored in the mp3 file. So the file is just a container for the recording. That container is designed in a specific way attuned to the characteristics and requirements of the data. Hence, mp3.
Similarly, that Jpeg or Raw is not your photo, it's a container designed to hold that kind of data. Aperture is all about the data and not about the container. What does that mean? Essentially, it means that (regardless of where you choose to store the master file) Aperture will manage the photo, edit the photo, add metadata to the Photo but never touch that file. If you choose to export - unless you specifically choose to export the original - Aperture will export the Photo into a new container - a new file containing the photo.
There are times when you need to get to a photo outside of Aperture - to upload, to share, to add to a document for instance.
There are many, many ways to access your photos that are managed by Aperture. Which to use depends on the purpose you have for the image. In general there are two main ways: Media Browsers and Exporting.
Media Browsers are all through the OS. They're in every Open... Attach... or Browse... dialogue, they're built into the tool bars of apps like Pages, iMovie, Final Cut X, Word, Mail and so on. Media Browsers access your Aperture Preview.
Note: That is not a Finder window. It's the dialogue you get when you go File -> Open... in an app...
Here's the Media Browser from mail
Note that the entire Library is available, note too the search box at the bottom which can be use to search on any text - titles, keywords etc -
The Media Browser yields your Aperture Previews. The quality of these can vary depending on the settings you choose in the Aperture Preferences. You must also elect to share the Previews with the iLife apps for the Media Browser to work.
There are limitations. There is no way to access a Raw file via Media Browsers, they are always jpeg, they have limited support for Exif and IPTC and so on.
Common uses for the Media Browser: when you need to use a shot in a word processing document, in a movie or for a handy email of a shot to family or friends.
Note: Drag and drop (or Copy and Paste) from the Aperture Window also gets the Aperture Previews.
There is a whole chapter in the manual on Exporting so I'm not going to recap that here. But there are some headlines to note, and common causes of confusion.
You can access the Export options by selecting a shot or shots and going File -> Export
You can export the Original file - that's the Raw if you're shooting Raw, or the unedited Jpeg. Basically, this is how you get out exactly what you put into Aperture.
You can export versions. These are your processed or edited shots. Have a look at the shot below
There are a lot of options. You can export to Jpeg, Tiff, PNG or PSD, in a variety of sizes and qualities, including metadata or not as you prefer. You can use the existing export presets or, at the bottom of the list you'll see the Edit... option. This allows you to create your own preset. So, if you regularly export to low quality Jpeg at 75% of the size, you can create and name a preset, and save it.
For instance, in the shot above you'll see the chosen preset happens to be JPEG-Rett Site - which is a created preset for a specific purpose.
Some common confusions:
Size refers to dimensions - length by breadth. So, a Jpeg at Original size will not have the same file size (or 'weight') in MB or KB as the original. It will be the same dimensions are the original, measured in pixels.
The File Size (or 'weight') in MB or KB is determined by the Image Quality - and that's an option you set in when you choose (or create) the export preset.
For more: RTFM - which stands for 'Read the FREE Manual'.
You can view the free manual online here
and download it as a pdf from here