In GNU/Linux, almost everything is represented as a file. This makes the system (fairly) consistent when dealing with hardware. Since (most) hardware is represented as a file, the system can usually deal with new hardware without the need for special software.
Files are organized into directories (also called "folders"). GNU/Linux has a standard, well-defined hierarchy for directories, with each directory containing specific files or kinds of files. The very top of the directory hierarchy is called the root folder, and is represented by a forward slash ( / ). Directories underneath the root folder are called "sub-directories". Sub-directories can have sub-directories, which can have sub-directories. You can have many levels of sub-directories.
The organization of directories and files is defined by the Filesystem Hierarchy Standard . This is a guideline to clearly explain what goes where -- and why -- so that the various version of GNU/Linux will operate in a mostly similar way. Remember, Ubuntu is just one version of GNU/Linux, there are many others! The purpose of the FHS is to ensure that what you learn about using Ubuntu will help you use other versions of GNU/Linux, too.
Files and directories have owners. The owner of files and directories can (usually) make changes to them. Your user account will usually own the files you create, but will not have access to most of the files on the system. Most of these files are required for the successful operation of the GNU/Linux operating system, and as a security precaution users are not permitted to modify them.
Most files and directories are visible to everyone -- that is, you can see that they exist, even if you can't necessarily see the contents of them. Some files and directories, though, are hidden. This means that they are not displayed to you by default. Hidden files and folders start with a period. There are a variety of reasons for having hidden files and directories, and the exact intent of each depends on the location of the item.
You have one directory over which you have full control. This is your home directory, and usually resides at /home/<username>, where "<username>" is the account name you used when signing onto the system. It is in your home directory that most of your files should be stored. Inside your home directory there are many hidden files and directories. Most of these contain configuration settings for the various applications you use. You own these files, and can read, modify and delete them. Most of the time, though, you don't want to access these files directly, and instead should let the application that created them manage them for you. It is for this reason that the files are hidden: they're there if you need them, but since you usually don't need to fiddle with them there's no reason to see all of those files on a regular basis.
Nautilus file manager
selecting, copying and moving files (p. 107)
bookmarks and file choosers (p. 99)
deleting files