Machine translation (MT) is the use of computers to translate from one human language to another automatically.
At the basic level MT performs simple substitution of words in one natural language for words in another. More advanced and useful MT adapts the translation to take into account the different grammars, idioms and other language artefacts between the two languages.
In the field of translation MT can be used either to pre-translate a document in which case the translator performs a editing function to correct the suggestions from the machine. Machine translation can also be accessed through Translation Memory, in this case the translator is not correcting MT translations but is able to use the MT suggestion if the translation is appropriate.
Both Apertium and Moses are examples of a open-source machine translation tools.
There are also web services that provide Machine Translation. Google Translate is an example of an online Machine Translation service. Google Translate is not an open-source tool.