is this a tool which is used frequently by standard linux users? should one search for a public key and add it to their public keyring for every download, or this generally reserved for special situations?

Not getting many responses, so I'm thinking I should rephrase my question or provide more information.

My impression of using this tool is only under very planned circumstances. The server has to have access to your public key to encrypt any information that it sends to you. So it seems like you have to configure it yourself, in other words, it has to be your server.

And as far as using this type of encryption with e-mail, is this only used when you have made agreements with others in advance? So, if I want to send encrypted e-mails to my friend Jack, I have to send him my public key first, and then he has to add it to his public keyring. Only then will we be able to send encrypted e-mails to each other.

So, again, use of this tool is very rare, unless it is specifically planned in advance between the parties.

Am I on the right track here?

I think you'll find that in most real cases it would be a one at a time thing ie you send your public key to whoever wants to communicate with you and then they can use it to encrypt a file.
In theory, you could have a public server holding public keys, but you'd have to trust that that server is trustworthy...
There is also the Enigmail extension for Thunderbird etc http://enigmail.mozdev.org/home/index.php

I don't want to try and recreate all of the documentation available on-line about public key encryption - I recommend a search for more in depth study. Here is my experience using gpg and Enigmail for Thunderbird.

First I installed Enigmail and gpg on my Linux machine. Then, using the wizard provided by Enigmail/gpg I created a key pair for my email account. This consists of two keys. A public key and a private key. Again using the wizard I published my PUBLIC key to a key server (pool.sks-keyservers.net) as recommended by the wizard. My private key is stored on my computer only and is protected by a STRONG passphrase.

I sent an email to my sister and signed it with Enigmail/gpg. When she received the email she used Enigmail/gpg to lookup my signature on the key server and to download my PUBLIC key. She created an email to me and encrypted it with MY public key. She also added her signature.

I received the email and used my PRIVATE key to decrypt it. I also looked up her signature on the key server and downloaded her PUBLIC key. I can now use her PUBLIC key to send her an email which only she can decrypt (using her PRIVATE key).

Now that I have figured out how to use it - to be honest I do not use it day to day - but I could if I wanted to.

If you have a couple of email accounts you can install Enigmail/gpg and create key pairs for both accounts. Then practice sending encrypted email between them. Copy a third email account on the emails and you will see that the body of the email is indeed encrypted (Enigmail will automatically decrypt the messages by default).

Hope this helps,

Ken