In order to encrypt emails to me (please send to opportunity@
If you're interested in learning more about encryption as it is effectively used to secure information with regard to protecting one's emails and other data from being exposed to those who can potentially otherwise be monitoring and intercepting your communications with others and gaining access to your data, the technical information on GPG is available at the GNU Privace Guard website. While there are many ways to encrypt data, what I'm attempting to provide information about here is technically referred to as assymetric encryption, which involves each person (algorithmically) generating their own keypair, which consists of a public key, shared widely with others, and a private key, which needs to be protected by not being shared with anyone else, and also almost always is protected by a passphrase (a sequence of keystrokes it would be hard for an adversary to guess) which the owner does not share (but must remember and/or keep secure/secret too).
Whether you agree with his choice to reveal the information he exposed or not, a fascinating story of how he put the technical knowledge he himself had about encryption and GnuPG keypairs to use in order to be able to public information he had access to as an employee with top secret clearance of a government contractor, by insisting that the journalists he was seeking to make contact with also engaged in a secure fashion by generating their own secure keyparis so that he was able to effectively share some initial information with them and only them, the book _Permanent Record_ by Edward Snowden provides quite a bit of context without overwhelming the non-technical reader with a great deal of confusing details (which is not true of the GnuPG website linked above, being its goal is to make the encryption software available to people who wish to protect themselves, which is inherently a complex topic that requires one to gain technical knowledge to a significant degree).
The Wikipedia Entry for Web_of_trust explains an awful lot as well, though again, due to the inherent complexity involved in effectively protecting communication channels and data from snooping by a motivated person or entity, the technical information present within that page may be daunting to people who aren't yet aware of all the different relevent aspects one should consider in order to have some confidence that one has succeeded at this goal.
Open standards are now the basis of all this, and the canonical reference can currently be found at the OpenPGP website. Just to make things even more complicated, the GnuPG developers have recently broken away from the consensus for the proposed direction of future OpenPGP standards, so it is possible that the coherence around a single way of doing all this which is easily interoperable by many different systems of encryption may not manifest as elegantly as some of us would like.
While the following explanation page is provided by a company trying to sell you their services, I find that it appears to be a fairly readable and accurate overview to provide background and may be of value to interested folks trying to understand more. Proofpoint's explanation of email encryption