19th
Projects and albums in Aperture
The primary organisation of images in Aperture is via projects (and blue folders containing projects). Every image belongs to one and only one project. Albums provide a secondary organisational structure which is independent of projects but can be freely combined with them.
The main difference is that the same image can appear in several albums. That means that dragging an image from one album to another, ‘copies’ the image, it does not move it (dragging from project to project moves the image). The same image can therefore appear in albums within different projects. And albums themselves can appear outside of projects. Pressing alt, while dragging from album to album moves the image, but only from album to album, the master still remains in its original project.
My advice would be to that any organisational criteria that is exclusive (eg, holiday locations or trips, customers) should be implemented via projects. Any organisational structure that is topical (eg, people vs. landscapes) should be done via (smart) albums. Any output oriented structure (eg, web site, book, presentation) is also best done via (smart) albums, with the build-in web galleries being just a special kind of albums.
Since every image has to be associated with a project, it can be that when topical and exclusive criteria lead to the same structure, a structure based on projects already provides the necessary topical division.
To summarize, first think what ‘exclusive’ organisational structure you want, ie, a structure that is unambiguous, that says this picture should be here and not anywhere else, and implement that structure using projects and blue folders.
Secondly, think about how else you want to access your images and use albums for that (and group with folders when necessary).
A lot of people choose the date as an unambiguous criteria. That is the lazy and unimaginative way (but it naturally works). I think it is worth to make the effort to create a more descriptive structure.