What about invoices?

Do you guys send invoices to your clients? I would feel better requesting my job or service's fee by isuing an invoice to the client. I've never seen an invoice, so I don't know how it would look like, and what to include in it. I basically to send invoices to clients asking for advertising/designing/hosting fees. If i get the general idea of how an invoice is like, I can edit it to cater for other use such as designing and hosting.

Can anyone help? Any help is appreciated. Thanks.