Every note placed on the staff needs to be assigned a rhythmic value.  The sum of the note values in a measure cannot exceed the Time Signature's beat declaration.  A note ultimately tells you two things.  It tells you what note you need to play and when and how long you should play that note.  This is one of the reasons reading music in standard notation is more difficult than tablature.  If you want to get serious with music though you will need to understand the various note values.  On guitar this is important when strumming as well since strumming is all about rhythm.

Parts of a Note

When you write a note on the staff it can contain different pieces of information.  Most of the note syntax has to do with it's specific rhythmic value.  The valid components of a note are the note head, the stem and the flag or beam.  A basic rule of thumb is "The more ink it takes to write the note, the shorter it's rhythmic duration is".

Below shows the different possible pieces of a note in standard notation.