A deal structure is the rule book or a fully defined step-plan of a transaction. Buyers and Sellers will agree on a certain transaction and define a catalog for capital and asset exchanges, conversions, actions, rights and options which have a clear function.
There are endless ways to structure a deal. The transaction parties could agree on selling a whole company, certain assets of a company (e.g. Patents), define mixed payment terms (exchange of shares and cash), define milestones for an earn-out – just to give you an idea.
In later stage transactions buyers and sellers will involve a number of professional advisors to structure and handle a deal. Usually this involves lawyers, tax-advisors, technical experts and a leading transaction advisor, who will manage the process and support his principal in transactions.