Actually, not a whole lot.
When the Canadian government decided on buying the McDonnell Douglas F/A-18 Hornet, it was largely assumed by the Canadian public that the Hornets procured for the air force would be modified in some way or another to further suit them towards the vital role they would play in defending Canadian sovereignty. The aircraft, criticized for their range (in comparison to other fighters like the F-15 Eagle and F-14 Tomcat, which were also part of the Canadian fighter replacement competition), were designed for naval operations, and though Northrop (later McDonnell Douglas, when they took over the production agreement) offered Canada a “de-navalized” F-18L Hornet, the Department of National Defence was very focused on buying the original naval variant.
Canada eventually received its full initial order of 138 CF-188 Hornets, as they are officially designated in Royal Canadian Air Force parlance, consisting of 98 single-seat CF-188A and 40 two-seater CF-188B units, completed in part at McDonnell Douglas facilities in Canada (which were built/retooled as part of the fighter procurement deal). By the end of production, however, 138 models were built; . Interestingly enough, the Canadian government only ordered two changes to the new fighters that would set them apart from their US Navy/Marine Corps F/A-18 Hornet counterparts. They decided to keep all of the features the Hornet possessed that allowed them to execute naval operations, including the tailhook, folding wings and reinforced landing gear. These two changes included a spotlight, mounted on the left side of the nose, and a “false canopy” painted underneath the fuselage of the aircraft.
The spotlight, fitted to the gun loading access hatch on the Hornet’s left side, was necessary for nighttime intercepts of Soviet bombers. The USSR (and even Russia today) had a habit of sending Tupolev Tu-95 “Bear” strategic bombers very close to Canadian airspace, both to probe Canada’s aerial defense capabilities so that the Soviet Air Force could remain up to date on Canadian response tactics, and, just to generally keep up the levels of occasional harassment the western and eastern sides of the Iron Curtain had gotten used to since the early 1950s. Should CF-188s be scrambled to intercept an inbound Bear at night, one fighter would fly closer to the large Soviet bomber, which typically would run with its exterior lights off, then turn on its powerful spotlight and positively identify the bogey, keeping in constant contact with the other CF-188 which would hang back, ready to fire a short range air-to-air missile should the situation deteriorate.
At the end of the Cold War in the early 1990s, it became less important for the RCAF to maintain these spotlights in the Hornet, and they were largely removed from most operational aircraft. However, a select batch of Hornets still retain their spotlights. These aircraft, known as “Q jets” form the RCAF’s quick response teams – small elements of fighters configured for air-to-air operations in the event that an airspace incursion needs to be addressed (i.e. a Russian Tu-95 flying near or in Canadian airspace). If you see a CF-188 that isn’t on Q status, you’ll find the spotlight’s plexiglass housing on the left (port) side of the fighter, but probably no spotlight inside.
The second significant change was a false canopy, painted underneath the fuselage of the Hornet, right below the cockpit. This was designed to add a degree of confusion in dogfights, temporarily disorienting enemy pilots who would hopefully look for a cockpit in determining which way the fighter would maneuver. The US Marine Corps and Spanish Air Force later adopted this paint scheme for their own Hornets; the US Navy refrained.
The remaining differences exist because of different operational requirements and capabilities between the air forces and wings that operate the Hornet. The US Navy, for example, equipped its Hornets with ROVER, short for Remotely Operated Video Enhanced Receiver, in 2006. This allows F/A-18C/D Hornets to receive live video footage from unmanned aerial vehicles (i.e. drones) in the battlespace while on-mission. Otherwise, the avionics and onboard sensor suite of the CF-188 are very similar to that of an F/A-18 Legacy Hornet.