A buy-write is a covered-call established in a single transaction rather than in two. The distinction is execution, not structure: you quote the combination on the complex-order-book and pay one net price, so you never own unhedged stock between the fills.
This matters more than it sounds. Legging into a covered call in a fast market can cost more than the call premium itself, especially in a wide-spread name. The package price also makes the arithmetic honest, because the net debit is your true cost basis.
Example: XYZ at $50.10, the 30-day $52.50 call at $0.95. Rather than buying shares and then selling the call, you send a buy-write at a $49.15 net debit for 100 shares plus one short call. Fill or no fill, you are never exposed to a gap between the two legs.
Related: covered-call, combo-order, complex-order-book, covered-call-fund