CAT, or Computer-Assisted Translation, Tools are software programmes used by some translators. They are not to be confused with Machine Translation - CAT Tools do not translate, they are merely programmes used to improve efficiency and consistency.
The programmes work by matching each sentence in the original document to its translation to build up a database. If words or phrases that have already been translated before are repeated, the CAT Tool will bring this to the translator’s attention and show how it was translated previously.
They can also highlight if the translator uses a different translation for the same term in the text. That is why it is beneficial to provide any glossaries or vocabulary that you may have and want the translator to use. The more informed the translator, the better the translation will be.