Colleen.

I wish you well with your business.

For example, if I designed an invitation in InDesign, what would be the
best way to present it to the printer if I wanted it printed in offset
lithography?




Generally, you can submit the complete InDesign files as is -- or better yet -- a high res PDF or PDF/X-1a file. But...speak to your printer and ASK what his file preferences are. In fact, you should have an open dialog with ANY printer you work with. It can vary from printer to printer. Don't worry about handing over actual color separations.

And can someone please tell me the difference between a printer and a
service bureau???




A service bureau was the way you went to have your files typeset or translated into film or to have slides scanned, color proofs pulled, etc., prior to handing them over to your printer. Service bureaus today seem to have moved over to digital (xerographic) color printing and large inkjet printing as film is infrequently required. Some print shops will function as service bureaus via their production departments.

But honest? I haven't used a service bureau in a few years. I create my own PDF/X-1a files for publications (advertisements), and turn over native files or PDFs to other printers.

Others may have different experiences.

Neil