Fred Francis invented Scalextric in 1952

They are similar, SCX is the equivalent of Scalextric in Spain. SCX cars will run on Scalextric track and vice-versa.

Technically yes, just the track is a different colour

Scalextric is a well-known brand of toy slot cars made for racing on a slot track. Micro scalextric are also slot cars, but ones that are more suitable for children than the full size Scalextric cars.

Scalextric Digital is basically the modern digital version of the Scalextric system to run on a digital track. Scalextric produce toy cars that run on an electrified track for children and car enthusiasts. The traditional Scalextric system was based on an analog track, but the new digital version has the ability to record lap times and other statistics.