The best way to understand the Linux permissions, is with an example. If we run the command ls -l we get an output like this:

1 2          3     4     5      6     7             8
- rw-r-----  1     user  users     8  ene 24 11:01  f.txt
d rwxr-xr-x  2     root  root   4096  ene 24 12:14  dir

which maps to

1 2          3     4     5      6     7             8
d uuugggooo  links owner group  size  date          name

Numbers from 1 to 8 are not really in the output, but are here to clarify every field.

1. First character specifies the type of the file. In the example above the hyphen (-) in the first character indicates that this is a normal file, and the d indicates that this other file is a directory. List with all file types
2. Next 9 characters specifies the files permission. Each 3 characters refers to the read, write and execute permissions for user, group and others. In this example, rw-r----- indicates read-write permission for user, read permission for group, and no permission for others.
3. Number of links, in this case is 1 for the file f.txt and 2 for the directory dir.
4. Owner, in this example, user for the file, and root for the directory.
5. Group, in this example, users for the file, and root for the directory.
6. Size of the file.
7. Last modified time.
8. File name.

Difference in access permission between files and folders are:

Remember, give execute permission to a folder if you want to access it.

Instead of use the values r, w and x, you can use numeric values, where

r = 4
w = 2
x = 1
- = 0

It is possible combine this values, a table with all the possibilities:

To change the permissions you can use the command chmod, which has this syntax:

chmod [options] MODE FILE

Let's see some examples, to give read and write access to everyone, you can use this two equivalent commands:

chmod a=rw file.txt
chmod 666 file.txt

To give read and write access to user and group, but no access to others, you can use this two equivalent commands:

chmod u=rw,g=rw,o= file.txt
chmod 660 file.txt

If you want to change the access permissions of a folder recursively never use the -R option. Chmod doesn't differentiate between folders and files, and this is really important, because execution access has different meanings for files and folders. Instead use this:

For directories:

find . -type d -exec chmod 755 {} \;

For files:

find . -type f -exec chmod 644 {} \;

To finish, if you want to change the file owner, you can use the command chown, with this syntax:

chown [options] OWNER[:GROUP] file.txt

For example:

chown user1 file.txt

To change the owner recursively, use the option -R If you just want to change the group, use the command chgrp.