Some commands have an effect on the next thing. If that next thing should consist of more than one character, these characters have to be grouped. Also, most commands appearing within a group lose their effect at the end of the group. Commands consisting of letters end with the first non-letter. If that non-letter is a space, the space is not introduced into the text.
Commands can also consist of a backslash followed by a single non-letter.
A blank line will start a new paragraph.