Competent cells are bacteria which can accept extra-chromosomal DNA. Cells can be made competent in several ways. One such way is to "shock" it, which involves heating the bacteria in a water bath at 42 degrees Celsius for 90 seconds, then placing it immediately in ice. This creates a thermal imbalance which allows extra-chromosomal DNA (such as plasmids) to enter the cell, allowing the bacterium to be genetically modifed.
See also: Transformation (genetics)