Detail what you will be providing (i.e. if a logo for instance say if you will be providing EPS files or if you will be doing three initial designs and then two revisions)
Detail and significant exclusions (i.e. what you will NOT be doing)
Say how long it will take you/timeline and any key delivery milestones (i.e. you will have initial designs in two weeks and then two weeks for revisions)
Most importantly your price for the work
Your payment terms and a copy of or a link to your standard Terms & Conditions
They may also like examples of similar work or a link to your website and you might even go so far as to say what will happen if the whole thing goes pear-shaped, i.e. how you will handle complaints/disputes.