In traditional notation, a tie is a curved line that connects two notes with the same position on the staff. A tie indicates that you play the first note only, and to hold it for the length of both notes. A tie is not necessary in tab notation where you can just follow the count.
A tie is necessary if a note is played over a bar line, as in the following two bar riff. When ties are used to connect quarter notes and eighth notes as in the following two examples, an "off the beat" feel is created. This is called syncopation.