Insertion Order for advertising

Ive been told to write out an IO for an advertiser-don't know what it is

[HostingDirectory]

I have recently been contacted by a company that wishes to advertise on my hosting directory. They have asked me to write up an IO ( Insertion Order ), so they can sign it and send it back. I have never writen one of these up before.
Can anyone explain to me in simple terms what i would need to include in the Insertion Order so that i can write it up.

Why is the IO so important to them? Is there a legal reason why they must have one? I live in the uk so the laws are different, i have never heard of an IO before.

I am completly confused about Insertion Order's in general.

What is the point of the Insertion Order?

Is it to protect the advertiser or the publisher. As a publisher do i really need my advertisers to fill out an Insertion Order?

Are they required by law to fill it out before they can recieve advertising from my site or am i required by law to make sure they fill one out?

To be honest i would rather not bother with an Insertion Order but need to know if advertisers will expect and want one or if i am required by law to ensure they fill one in?

If so, would an Insertion Order be filled in before a payment is recieved or after a payment is recieved?

I plan on taking payments for advertising through my website with paypal so an Insertion Order complicates things.

Any advice and help appreciated.

[ogletree]

Go to Google and type in Insertion Order. You should find all kinds of things to help.

[Compworld]

A insertion order is like an invoice for advertising. It usually contains your company name and information, their information, what they are buying, terms and conditions, and two signature lines for both parties (title, name, date, sign.). You can probably pull a sample from goole by typing sample insertion order.

Hope that helps,

CompWorld

[HostingDirectory]

I already searched google for information on it but was not find anything to answer my questions. That is why i posted here.

I did discover a basic outline to what an IO is but nothing beyond that.