Almost completely correct Tom.
Your camera makes files that are 2x3. That only matters if you're not cropping the images. You can crop them however you like, and Lr offers several default options there, 2x3, 4x5, 5x7, 1x1, and more, and the ability to add custom choices as well. The main point is that the crop you apply needs to match the intended output. Suppose you took a very nice picture of a flag pole. Perhaps, using it on a website, it would make a nice element on the side of the page which is 1 unit wide and 1' units tall. That could look quite nice. But should you send it to a printer, something's going to have to give. There's just no way to make a 1x1' fill up a 4x6 sheet of paper, without distorting the image. Wedding photographers (among others) have to consider this when planning their shots, because zooming in on a group photo so that the group fills the width of the photo, can have dire consequences if it has to be cropped to fit enlargements. (Bye-bye, new in-laws
)
The lock icon forces the aspect ratio to remain the same as you crop, that is, no matter what portion of the crop box you drag, the rectangle remains the same shape proportionally. If you unlock the icon, dragging a corner keeps the same ratio, but dragging an edge causes the box to get taller/shorter or skinnier/wider. Normally if you choose an aspect ratio it should remain selected as you move from image to image, but there have been several bugs in the past uncovered where this setting wasn't sticky. I think it's working right in 2.6, but.....
Files are the actual images from the camera. Lr needs these to function, and you need to make sure they're backed up.
Folders are the OS's way of organizing the files. Lr doesn't need to organize files in folders, but most people do. This is subject to much debate between Lr users, principally concerning how much time to spend/waste, (depending on your viewpoint) in organizing folders. The general consensus is to organize the image files within OS folders by some form of date structure. Lr provides the tools to do that at import time. Other more complex organization would be handled within Lr.
Catalogs are Lr's fundamental data structure. Within the catalog, which is fundamentally a database file, the filename/folder location of the image files are tracked, any changes you've made to the images are tracked, and any additional management/organizational tools you've used are also tracked. The actual images themselves are not stored in the catalog, just the links to them where they're stored on your PCs drives.
Collections are a virtual way of organinizing your photos within Lr. They allow you to create ad hoc groupings of images, based on a number of parameters, without actually creating additional copies of the images. Many people liken Collections to Playlists for mp3 players/iPods. You may have a shot of a family member, that could fit simultaneously into collections such as 'Family', 'Margie', 'Blonde', 'Red Sweaters', 'Laughing', 'Christmas < Holidays < Events'. That would be almost impossible to manage with Windows Folders, but is trivial using Lr collections.
You can assign keywords, numerical ratings, pick/reject flags and other metadata to each image, and create collections to track all or some of these.
These are good questions that you've asked, and I've waffled on quite a bit, If I haven't answered your questions, I hope I've at least pulled you down the road to new questions.
