Machine translation (MT), is a sub-field of computational linguistics that investigates the use of computer software to translate text or speech from one natural language to another. At its basic level, MT performs simple substitution of words in one natural language for words in another. Using corpus techniques, more complex translations may be attempted, allowing for better handling of differences in linguistic typology, phrase recognition, and translation of idioms, as well as the isolation of anomalies.

Current machine translation software often allows for customisation by domain or profession (such as weather reports) — improving output by limiting the scope of allowable substitutions. This technique is particularly effective in domains where formal or formulaic language is used.
Computer-assisted translation, computer-aided translation, or CAT is a form of translation wherein a human translator translates texts using computer software designed to support and facilitate the translation process.

Computer-assisted translation is sometimes called machine-assisted, or machine-aided, translation.

Computer-assisted translation is a broad and imprecise term covering a range of tools, from the fairly simple to the more complicated. These can include:

• Spell checkers
• Grammar checkers
• Terminology managers, allowing the translator to manage his own terminology bank in an electronic form.
• Dictionaries on CD-ROM, either unilingual or bilingual
• Terminology databases, either on CD-ROM or accessible through the Internet
• Full-text search tools (or indexers), which allow the user to query already translated texts or reference documents of various kinds.
• Concordancers
• Bitexts, a fairly recent development, the result of merging a source text and its translation, which can then be consulted using a full-text search tool.
• Project management software
• Translation memory
• Systems that are nearly automatic as in machine translation, but allow user decisions for ambiguous cases. These are sometimes called human-aided machine translation.

Although the two concepts are similar, computer-assisted translation should not be confused with machine translation (MT).

In computer-assisted translation, the computer program supports the translator, who translates the text himself, making all the essential decisions involved, whereas in machine translation, the translator supports the machine, that is to say that the computer or program translates the text, which is then edited by the translator, or not edited at all. Difficulties with such unedited output are described at machine translation.

Information retrieved in: Wikipedia