A backcross is a hybrid plant that has been bred with one of its parents (or a plant that is genetically similar) in order to create offspring that is closer to that of the the original parent. For example, a grower could breed a plant with its own father to make sure the baby has its dad's height. This is often done to maintain rarer strains or strengthen those with desired recessive genes.
BHO stands for butane hash oil and is a potent concentrate of cannabinoids made by dissolving marijuana in its plant form in a solvent (usually butane). The resulting product has very high THC levels (generally more than flowers or hashish) and is a thick, sticky oil. BHO is also referred to as honey oil, "dabs" or "dabbing," earwax, or shatter, depending on the manufacturing method.
Bud refers to the actual flower of the marijuana plant. These are the fluffy parts that are harvested and used for recreational or medicinal purposes as they contain the highest concentrations of active cannabinoids.