Difference between Folders and Collections

I'm a bit late to the party but a useful difference is that image files are held in a single folder but can be in multiple collections/albums. For example a collection called 'cars'  can have all images of cars but an image of a Ford can also be in a catelogue called 'Ford', and a catelogue called 'Blue' etc, etc. Whereas the image of a blue, Ford car can only be in one folder. 

dustins8429052  wrote Why I would work with collections if I have a nice folder structure in my HDD?

Collections are different than folders. They can contain photos from many folders. They are simply a list of photos, which LR can use for specific  tasks. The only time I use collections is to produce things like slideshows or photo books or similar. Generally, I would not use collections to organize, for pure organization I use keywords and other metadata.

Since you ask the question, and since you state you have a nice folder structure, the whole point of LR is to give you tools that are more powerful and flexible for organization and searching than the folder structure. If all you use is folders for organizing, you can't really search and find all pictures of a particular subject matter (for example, all photos of your children, or all photos of mountains) using folders alone, you need keywords and other metadata assigned to the photos, and then such searching becomes very simple.

Smart Collections are working directly related with the keywording system?

This isn't really a question (despite the question mark at the end), it is a statement which is sometimes true, and sometimes false. Smart collections can work with almost any metadata to provide a way of finding photos that have certain common characteristics.

Collections are just LIST of images. Like if you wrote down all your Record Albums or CDs on a piece of paper. That piece of paper doesn't have your Records or CD in it.

Difference btwn smart previews and 1:1 previes?

Previews are essentially jpgs created by LR for display in the Library module; 1:1 previews are the same size as the original and therefore useful for examining the image display at 1:1 zoom, but are not needed otherwise. Smart Previews are dng files that are no more than 2560 pixels on the long side and can be used by the Develop module as surrogates for the originals when they are not available - as, for instance, when the original image files are on an external drive at home and you are in the field with your laptop. If the originals are always available, SPs are not needed.

Folders are physical- they exist on your hard-drive. They contain your photo files.

Collections are Virtual- they are only a list of photos that you have gathered together for a special purpose in Lightroom.

Some people explain Collections like "Play-Lists in iTunes" or a "Grocery List" of items you want to look for in the Store.

Youtube is replete with Lr videos, some quite extensive.

Chris Orwig, Matt Kloskowski, Terry White, Julieanne Kost would be a fair start.

Lightroom Killer Tips

Dozens of Lr tutorials at jkost.com  <click the Lightroom tab>

Then, of course, there is Lynda.com. Hours of tutorials on the subject.