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If Squid gets a request and the http_access rule list gets to a proxy_auth ACL or an external ACL (external_acl_type) with %LOGIN parameter, Squid looks for the Authorization: header. If the header is present, Squid decodes it and extracts a user credentials.
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NOTE: The name and password are encoded using "base64" (See section 11.1 of RFC 2616). However, base64 is a binary-to-text encoding only, it does NOT encrypt the information it encodes. This means that the username and password are essentially "cleartext" between the browser and the proxy. Therefore, you probably should not use the same username and password that you would use for your account login.
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In addition to the well known Basic authentication Squid also supports the NTLM, Negotiate and Digest authentication schemes which provide more secure authentication methods, in that where the password is not exchanged in plain text over the wire. Each scheme have their own set of helpers and auth_param settings. Notice that helpers for different authentication schemes use different protocols to talk with squid, so they can't be mixed.
http://wiki.squid-cache.org/Features/Authentication#How_does_Proxy_Authentication_work_ in_Squid.3F