It comes in a TO-220-3 through-hole package, the workhorse three-lead format for offline power supplies, flyback converters, and PFC stages.
ST officially lists the STP5NK65Z as Obsolete. For a board that needs this exact part — say, a repair of a power supply that scorched the original — the channel is surplus and broker distribution.
The 650 V Vdss gives you headroom for 400 VDC bus rails (typical in power-factor-corrected offline supplies) with derating for ringing and transients. The 5 A continuous rating is at the case temperature of 25°C — real-world derating at elevated case temperatures will reduce that figure, so check the thermal curve in the datasheet. Maximum power dissipation is 85 W at the case — a heatsink is mandatory for any continuous load above an amp or two.
The TO-220 package is through-hole, which simplifies heatsink attachment and is a common footprint on existing boards. If you are reworking a board, the three-lead TO-220 is straightforward to desolder and replace — just watch the thermal pad and mounting torque.