Hi Kyra,
There are a few types of Tokens.
Lead tokens are basically place holders in your email that will be filled with information about the lead you send the email to. For example you could have one email with the {{lead.First Name}} token go to 10 leads and the token would fill with the first name of each lead respectively.
System Tokens do the same thing but with date and time info mostly.
Company tokens are for information about the company/account the lead is associated with.
My Tokens are defined by you and are local to a Marketo program. They can only be used in assets that are local to that program. Many users will create a program using tokens in all their emails and campaigns so that they can clone it again and again for similar programs.
Does that answer your questions?
John