An offer letter is a short document confirming the basic terms of a job (title, compensation, start date), typically not a binding, comprehensive contract. An employment agreement is a more detailed, binding contract covering things like confidentiality, IP assignment, and termination terms.
Most startups hire on an offer letter alone for standard roles, paired with separate standalone agreements (an NDA, an IP assignment agreement, sometimes a non-compete where enforceable) rather than one long combined employment contract. A full employment agreement is more common for senior executives, where specific terms like severance or a longer notice period are being separately negotiated.
Whichever format is used, the IP assignment and confidentiality provisions matter regardless of the label on the document. A short offer letter with proper attachments protects the company just as well as a long employment agreement, as long as the actual substance, who owns the work, what's confidential, is covered somewhere and actually signed.
Add your revenue and website for a full diligence brief, reviewed by a CPA who ran EY's West Coast R&D Tax Credit practice for 13 years.