A certified English translation for use in the United States must be prepared in accordance with the United States regulation, 8 CFR 103.2 (b)(3) and may be prepared by a professional translator or translation company. Other organizations, such as WES and various universities and medical schools sometimes have additional requirements, but basically a certified translation is made up of four parts which must be in place in order to be considered a valid certified translation.
A statement signed by a Notary Public is nice to have but optional for the USCIS, but some other organizations require it.
A translator or translation company does not need to be certified in order to prepare a certified English translation.
What is a certified translator?
Unlike most other countries, in the United States there is no federal or state licensing or certification for translators. There are credentials available to translators working in some language pairs in this country, but they do not carry the same weight--in the market place or in the translation community--as federal licensing or certification in other countries.
The American Translators Association (ATA) offers translator certification in some language pairs. ATA certified translators are required to specify the language pairs and directions in which they are certified. For example, a translator certified to translate from Spanish to English is not necessarily certified to translate from English to Spanish.