home - This directory is used as a starting point for users home directories. These are directories that are given to users for them to store any files they create. The users directory is a subdirectory to home normally the same name as the username.
usr - This directory is used to store installed user applications.
var - This is used for files of variable length. Files that change a lot such as log files would be kept here.
tmp - If used this directory is used for temporary files that are only needed for a short period of file. Some systems use housekeeping jobs to periodically delete files in this directory so important files should not be stored here.
etc - This directory holds many of the configuration files used by applications.
bin or sbin - Whilst you may see bin or sbin directories at the top level these are normally symbolic links to the directories /usr/bin or /usr/sbin respectively.
There are two other top level directories that are used as starting points for other directories.
dev - This is used to hold the devices available on the system. For example the first hard disk (hdisk0) can be referred to as /dev/hdisk0
mnt - This is the mount point which is used as a convenient place to attach other devices such as floppy disk drives and CD-ROM Drives. For example the CDROM drive may be mounted as /mnt/cd0/ although the name does not have to be the same as the device name.
These then subdivide into further directories which can be represented as a tree.