Some will transcribe your spoken words into a different language and then speak the result to you.
Others are less detailed and better for simple word-to-word or website translations.
Google Translate: Best Overall Translator
Works quickly.

cybrain / iStock / Getty Images Plus
Translates text, images, documents, and websites.
Supports a huge variety of languages.
Can read the translation aloud.

It’s been known to make very wrong translations.
Just key in or speak, and then watch the translation appear off to the right.
you’re free to key in text, speak it, or use an on-screen keyboard.

Any word you highlight in the input text box shows definitions, example sentences, and translation information.
Other features Google Translate offers:
Microsoft Translator: Best for Live Conversations
Really easy to use.
Supports lots of languages.

Only translates text (not images, websites, etc.)
Can add translations to your list of favorites.
A direct link to the translation can be shared with anyone.

Checks spelling as you throw in.
OCR isn’t perfect.
Yandex Translate is an absolute beast.
It’s really helpful for one-time lookups but is also nice to use for learning a new language.
Here are some other features:
Reverso: Best Language Learning Translator
Has a spell checker.
Translates without having to click a button.
Listen to the source and translated text.
Bookmark translations and learn new words with flashcards.
Support for relatively few languages.
Instant translations are often slow.
Like Google Translate, Reverso translates between languages automatically and supports several of the more common languages.
Something worth mentioning about this website is the context translations it offers.
you might also rephrase your input or output text for better readability with a few clicks.
Much like other translation sites, this one lets you bookmark your favorite translations.
There’s also a vocabulary section that can help you learn new words through flashcards and word lists.
Internet Slang Translator: Best Informal Translator
Conversions happen automatically (no button-clicking needed).
Translates common internet lingo.
Lots of slang terms don’t translate appropriately.
Some regular words are translated incorrectly.
As the name would suggest, Internet Slang Translator is more for fun, not practical use.