Transformation of a multicellular organism
Your inference is basically correct. The Agrobacterium acts as a vector for a plasmid containing the genetic construct one wishes to introduce into the plant genome. A medium containing a large number of bacterial cells transformed with this plasmid is injected into plant tissue, and the plasmids enter plant cells where parts of their DNA are integrated into the host genome. The overall effect of this is that there is localized integration in the areas where the Agrobacterium was introduced. A nice visual illustration of this comes from a paper from our lab where my colleagues introduced the genes for betalain biosynthesis into the tissue of Nicotiana benthamiana: in Figure 3 you will see only a localized area of cells around the injection site gain the ability to produce betalains.
You can find a nice review of the whole process of transformation with Agrobacterium here.