Learn English – “Insert your data” or “Enter your data”

Fill means to add content to the container or gap until it is full.

In particular, "filling" tends to involve a physical action, such as filling a mug with water, or filling a form in with a pencil.

Please fill this jug with water

Please fill in this form.

I have filled in all of the empty days on the schedule with stuff to do.

We need to fill the hole in the road with new gravel

We need to fill the vacancy in sales before Tuesday.

Fulfil on the other hand means to complete an action or requirement. In particular, one tends to fulfil metaphorical or non-tangible things, such as requirements, objectives or promise.

You need to fulfil all of the requirements before the end of January.

Fulfilment of the key objectives is our primary goal.

I'll fulfil my promise to help you with your homework before it's due in, really I will!

Fill in is used when you tell someone to enter something (one field) on a form. Example:

Fill in your name here.

It could possibly be more than one field, but generally not the entire form. Example:

Fill in your name here, and your phone number over here.

Fill in means to supply something that's missing. So you fill in the blanks on a test, for example, or you can fill in a triangle with a color (change it from an outline to a solid triangle by coloring inside it), or you can fill in (substitute) for an absent colleague at work. And you can fill in a form because you're supplying missing information.

Fill out is generally used when you tell someone to enter all the fields on a form of more than one field. Example:

Please fill out this form.

Fill out means to complete by supplying requested information.