Recombinant plasmid is an example of an application of bacteria or virus?

A recombinant plasmid is an example of use of a bacterial cell. The way we get to it does involve viruses, but indirectly; the restriction enzymes used to produce the recombinant DNA come from the bacteria's own defense against viruses.

Genetic engineering involves using bacterial cells to produce a product for which bacteria have no use, but which humans want. We essentially force bacteria to produce things such as human insulin and enzymes that help...

A recombinant plasmid is an example of use of a bacterial cell. The way we get to it does involve viruses, but indirectly; the restriction enzymes used to produce the recombinant DNA come from the bacteria's own defense against viruses.

Genetic engineering involves using bacterial cells to produce a product for which bacteria have no use, but which humans want. We essentially force bacteria to produce things such as human insulin and enzymes that help plants resist insects and viruses. In order to do so, we utilize bacterial plasmids. These are small, circular pieces of DNA in bacteria. In nature, bacteria make use of enzymes called restriction enzymes to cut apart DNA from viruses that are trying to attack them. Scientists make use of restriction enzymes in order to cut open plasmids at a very specific sequence of base pairs. As all DNA contains the same four types of bases, the restriction enzymes can also be used to "cut out" sequences from the DNA of other organisms, that code for proteins we would like the bacteria to produce. When the plasmids are mixed with the foreign DNA, some of the desirable foreign DNA is incorporated into the plasmid DNA. When the plasmids are reinserted into living bacterial cells, the foreign DNA is reproduced along with the rest of the plasmid when the cell replicates. The foreign gene can then produce the protein we want. 

There is a lot more to this process, of course; see the links below for more detail. The basics can be and are done in high school biology classes. Where I work, we use this process in Biology 3. The gene that makes some jellyfish glow in the dark is introduced into bacteria--which then glow in the dark.