A task list is all the steps the user must go through in order to achieve her goal.
If your goal is to write an email, then the task list shows all the steps you’d take in order to do that. Open your email application, select a new email, write the body of the email, write the subject line, insert the email of the person you’re sending it to, and then click send.
Designers think about the functions an application will need by thinking about the user’s goals and the tasks she’ll need to complete.
When you design in terms of tasks and goals, you ensure that you haven’t forgotten anything critical (like sending the email before an email address has been added). Task lists are usually linear in that one thing has to happen before the next. Or a set of things (writing the email, included a subject line and email address) have to happen before the next set.
Task lists are one of those things that seem simple on the face of it, but end up being complicated and often confusing.
For instance, are you required to write the subject line in order to send the email? (In some applications, no.) Do you have to write the body of the email? (Again, in some apps, it is not required.)
Can you send an email that has no subject line and no content in the body? These are all questions the product designer had to think about and make decisions about.
Some tasks impact others — you can’t send an email if there’s no email address to send it to — while other can happen at any time, or don’t have to happen at all.
Here’re are those same email-writing steps as a task list:
Sometimes we write these on post-it notes and stick them to whiteboards so we can move them around. (Because product designers do love our post-it notes.)