What is Proxy ?

A proxy is a software agent (a computer system or an application program) that acts on behalf of a user. It acts as a mediator for requests form clients to servers. A client connect to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a server. The job of a proxy server lies in evaluating the request according to certain filtering guidelines. If the request is validated, the proxy will connect to the relevant server and request the service on behalf of the client. The proxy server may optionally alter the clients request or the server's response. It may also cache the response so as to reuse the same for similar future requests.
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the internet.

Security Benefits of a Proxy :-: OrAdvantages of Proxy :-:

Usage of proxies can help in securing the communications over network in the following ways :=:

(a) It can help keep the server anonymous from client and the client anonymous from server. The server would not know whose request it is serving and the client would not know who has responded to its request.

(b) It can help apply access  policies to network services or content. For example, we can block undesired sites by creating the appropriate filter.

(c) Since all requests will pass through the proxy serevr, it can help log / audit usage. For example, a company can use a proxy to get a report on the internet usage of its employees.

(d) It can help scan transmitted content for malware before delivery.

(e) It can help scan outbound content, e.g. for data leak protection.

In addition to above benefits, there is a downside to it as well. Proxies can by pass security / parental controls. For example --
A proxy server that passes requests and replies unmodified is usually called gateway or sometimes tunneling proxy. 

Reverse Proxy :-:

A reverse proxy is (usually) an Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
When secure web sites are created, the SSL encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. A host can provide a single SSL proxy to provide SSL encryption for a number of hosts; removing the need for a separate SSL Server Certificate for each host. However, this has the downside that all hosts behind the SSL proxy will have to share a common DNS name or IP address for SSL connections.

Also See :-:
                    What is Cryptography ?
                    CSMA/CD
                    Types of computer attacks.