Switcher = lightweight, low-horsepower (around or under 1000hp) locomotive with small fuel tank and low-speed trucks, primarily used for shunting cars in a yard or making local deliveries to industrial spurs. These include the Alco S-series, EMD SW and NW series, GE 44- and 70-tonners, etc.
Road switcher = medium weight, medium horsepower locomotive with larger fuel tank and trucks capable of mainline road speeds, mostly used for local trains delivering cars to industries along a rail line, but can be used in a pinch for helping haul mainline trains. These include locomotives like the 2000hp EMD GP38-2, GE B23-7, etc.
Road freight locomotive = heavyweight, high-horsepower locomotive with very large fuel tanks for long range running, usually with a turbocharger and trucks geared for fast running, majority of which has 6 axles. These include locomotives like the 3000hp EMD SD40-2, 4400hp GE AC4400CW, etc.
Passenger locomotive = heavyweight, high-horsepower locomotive with very large fuel tanks for long range running, usually with a turbocharger and trucks geared for VERY high speeds, majority of which has 4 axles. These include locomotives like the 3000hp EMD F40PH, 3200hp GE P32AC-DM Genesis, etc.
Hope this helps.