Basic Information
The explosive round is a round of ammunition which relies on its explosive content for its primary effect. This type of ammunition is generally found in weapons of above small arms calibre since smaller rounds rarely carry enough explosive to generate a significant blast and are better classed as self fragmenting rounds. Obviously grenades tend to be explosive as well.
Explosive rounds are good for attacking unarmoured targets, unfortified buildings and terrain features (including earthworks) - harder targets tend to cause them to waste most of their energy bursting on the outside.
The explosive round begins with black powder - this being the first practicable explosive available to humanity - and pretty much stayed there until the developement of stable high explosives in the late nineteenth century: dynamite turned out to dislike the shocks inherent in being fired. Once the modern explosive filler had been developed, there was a rash of different recipies for explosives and mixtures of explosives every manufacturer (not to mention customer) is liable to have their own preferred composition. Technically, you could probably have encountered high explosive rounds for a catapult if someone was prepared to load a barrel of gunpowder onto one. There is little historical evidence for these and when they do occur they tend to be incendiary rounds instead.
Fusing of explosive rounds is also prone to vary - the time fuse is probably the oldest, beginning with a length of burning fuse match and evolving from there. The impact fuse took longer to develop (or at least to develop in such a way that it could be handled and fired safely) and proximity and altitude fuses are fairly recent, dating from late WW2 onwards. Combination fuses - such as delayed impact (impact + timer) are also popular for specific jobs.
As noted above, the explosive round relies on its explosive content - primarily the blast effect - with the generation of high velocity debris (confusingly referred to as shrapnel1) as a bonus. If the round has additional fragmentation materials built in, it may well be called high explosive/fragmentation or HEF - otherwise, explosive rounds are just as likely to be called 'general purpose'. There is also a modern trend for the use of HEDP (high explosive/dual purpose) - shaped charge rounds with an added burst effect. Where fragments are not desirable, explosives may also be cased in paper or plastics.
These are normally solidly military equipment - very few goverments are happy to see their citizens in possesion of such things.