In order for a cell to be transformed by a virus, the viral DNA must be entered into the host cell.  The simplest consideration is viral transformation of a bacterial cell.  This process is called lysogeny.  As shown in Figure 2, a bacteriophage lands on a cell and pins itself to the cell.  The phage can then penetrate the cell membrane and inject the viral DNA into the host cell.  The viral DNA can then either lay dormant until stimulated by a source such as UV light or it can be immediately taken up by the hosts genome.  In either case the viral DNA will replicate along with the original host DNA during cell replication causing two cells to now be infected with the virus.  The process will continue to propagate more and more infected cells.  This process is in contrast to the lytic cycle where a virus only uses the host cells replication machinery to replicate itself before destroying the host cell.
 The process is similar in animal cells.  In most cases, rather than viral DNA being injected into an animal cell, a section of the membrane encases the virus and the cell then absorbs both the virus and the encasing section of the membrane into the cell.  This process, called endocytosis, is shown in Figure 3.