What is horizontal gene transfer in bacteria?

It is the process by which DNA is transferred by a bacterium to another bacterial cell which is not its offspring. An artificial way of doing it is to transform bacteria with some DNA: researchers do it every day. Another way you probably heard about is the conjugation

What is horizontal gene transfer in bacteria?
Horizontal gene transfer occurs when genetic materials are transferred among bacteria using the three mechanisms: transduction, transformation, and conjugation. Contrast this with vertical gene transfer that is from parent to the offspring. In vertical gene transfer the whole genome is transferred as the bacteria divide themselves to give identical copies. While in horizontal gene transfer (lateral gene transfer) mostly only part of the genome is transferred or even only the F plasmid is transferred (F plasmid is separate entity from the bacterial genome and contains mostly only factors that enable its transfer, not the traits of the bacteria). These transferred segment may recombine with the genome inside other bacteria that receives it.