These are not at all the same and in fact serve very different purposes.
While it is true that in certain situations, a so-called “straight” translation is not appropriate (such as in advertising, for example), it is not true that all good translations are in fact adaptations. In reality, a good translation is NOT an adaptation. A truly good translation must remain faithful to the full context of the source text in terms of meaning as well as style, appearance, register and message. The words used to convey it are as important as the message, and while of course one must make allowances for what the reader will or will not understand in the target language, the translator really cannot be permitted to take “liberties” with the text. If it is targeted to a particular audience and is written in a particular register in the source language, it must be targeted to the corresponding audience and written in the corresponding register of the target language.
An adaptation, on the other hand, takes the ideas of the source text and re-writes them in a completely new way. The source text may be altered somewhat to appeal more to a new audience (i.e.: different marketing sector, class or age group for example) or it may be placed in a different setting. Adaptations are more common in literary, poetic or advertising media, where you can choose to forgo either media (form) or literal meaning in favour of conveying a particular message or emotion, if one or the other is considered more important to the individual situation.
Before deciding how much adaptation is necessary, the translator has to consider the purpose of the document in terms of its use and audience. For example, a letter translated for use in court must say exactly what is said, with no changes made to the message or the medium. The same letter, if it is to be sent to a potential client or political ally, for example, may need to be adapted somewhat, as the format of a letter in French is often different from the format of a letter in English (different greetings, different way of signing, different way of writing the address) and paragraphs may even be rearranged to place focus on the same notions but in a manner or order that is more appealing or persuasive in the culture of the reader, so as to make a sale or an ally.
The same kind of decision can be made on whether to translate or to adapt a piece of literature. For example, “Romeo et Juliette” is a translation meant to introduce Shakespeare’s “Romeo and Juliet” as written, but to a French audience, while “West Side Story” is an adaptation that creates a new version of the same story, but with a twist that is meant to appeal to a 20th century American musical-loving audience, different from its 16th century English theatre-going counterpart. Both, however, are equally good but serve different purposes.
Most “translated” poetry is also the result of adaptation and not translation, because it is nearly impossible to convey the same emotions to readers of another culture while retaining both the same form and the same words as the source poem. Poetry, like advertising, is very personal and highly culture-oriented; metaphors change from culture to culture, as do stylistic preferences, which is what poetry is often all about. This does not mean that poetic translations are never done, but they are extremely rare.
A related notion is that of localization. This is where this concept gets tricky, because while localization often involves translation, it belongs to a very specific modern reality. Localization is the process used to adjust a product or service (usually software and websites but it can also include products that come with a lot of manuals and accessory packages) to a particular language, culture, and desired local “look-and-feel.” In localizing a product, in addition to idiomatic language translation, such details as time zones, currencies, national holidays, local color sensitivities, product or service names, gender roles and geographic examples must all be considered. A successfully localized service or product is one that appears to have been developed within the local culture. (But remember that the same can often be said for a translation or an adaptation, as well.) Localized texts include texts that may have to be produced several times in the same language, but adjusted for dialectal and other cultural differences (lift versus elevator, metric versus imperial measurements, etc.), or texts that specifically target one area where that language is spoken (e.g. U.S. versus U.K., Quebec versus France). This is not, however, an adaptation, because the same content and message are generally still expressed in the same way, and such products are often designed to be easily localized without needing to alter the format, style or imagery.
Meanwhile, it is not only translations of scientific and legal texts that require faithfulness to the text – often referred to as “straight” translations (note that this does not mean “word-for-word”). Newspaper articles must retain all the same facts and be addressed to a corresponding audience in the target language community. Government documents, corporate literature, public information booklets, travel guides, textbooks and many other types of texts have to retain the same content, same register, same style and same format when translated even while respecting the structure, grammar and cultural baggage of the target language. Otherwise, you no longer have a translation but have moved into the area of adaptation.
In brief, a true translation must be written in a manner that is natural and appropriate to the target language, but may not diverge from the essence of the source text; nothing may be added, deleted or in any other way altered from the source. A true adaptation is a re-invention of the message to suit a new audience, whether that be a new language or different age or cultural group, modern vs. previous era, etc.