The signature of a person on a contract or on any other document involved in any form of transaction between two or more parties is considered be an absolutely valid Halachic Kinyan. A person who has signed such a document may not back out of the agreement that he has signed without the express permission of the other party involved.
The signature of a person on a document constitutes agreement to all terms mentioned in the document, even if witnesses testify that the signer did not read or understand all of the terms mentioned therein. Therefore, before signing any document a person should carefully examine everything mentioned in it, or ask someone whom he trusts to explain to him what exactly it is that he is signing to.
The Halachos discussed above are stated explicitly in the Shulchan Oruch (Choshen Mishpat 69:1), in the Shach there, in the Nesivos in Biurim, and in the Teshuvos HaRashba (Siman 77).
(This answer assumes the digital EULA\Terms and Conditions is a valid halachic contract, and the checkbox is a valid halachic signature. Whether this is true is asked here and here.)