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A reverse proxy server is a type of proxy server that typically sits behind the firewall in a private network and directs client requests to the appropriate backend server. A reverse proxy provides an additional level of abstraction and control to ensure the smooth flow of network traffic between clients and servers.
What is a reverse proxy? As its name implies, a reverse proxy does the exact opposite of what a forward proxy does. While a forward proxy proxies on behalf of clients (or requesting hosts), a reverse proxy proxies on behalf of servers.
A VPN is quite similar to a proxy. Your computer is configured to connect to another server, and it may be that your route web traffic through that server. But where a proxy server can only redirect web requests, a VPN connection is capable of routing and anonymising all of your network traffic.
The previous answers were accurate, but perhaps too terse. I will try to add some examples.
First of all, the word "proxy" describes someone or something acting on behalf of someone else.
In the computer realm, we are talking about one server acting on the behalf of another computer.
For the purposes of accessibility, I will limit my discussion to web proxies - however, the idea of a proxy is not limited to websites.
Most discussion of web proxies refers to the type of proxy known as a "forward proxy."
The proxy event, in this case, is that the "forward proxy" retrieves data from another web site on behalf of the original requestee.
For an example, I will list three computers connected to the internet.
Normally, one would connect directly from X --> Z.
However, in some scenarios, it is better for Y --> Z on behalf of X,
which chains as follows: X --> Y --> Z.
Here is a (very) partial list of uses of a forward proxy server:
1) X is unable to access Z directly because
a) Someone with administrative authority over X's internet connection has decided to block all access to site Z.
Examples:
The Storm Worm virus is spreading by tricking people into visiting familypostcards2008.com, so the system administrator has blocked access to the site to prevent users from inadvertently infecting themselves.
Employees at a large company have been wasting too much time on facebook.com, so management wants access blocked during business hours.
A local elementary school disallows internet access to the playboy.com website.
A government is unable to control the publishing of news, so it controls access to news instead, by blocking sites such as wikipedia.org. See TOR or FreeNet.
b) The administrator of Z has blocked X.
Examples:
The administrator of Z has noticed hacking attempts coming from X, so the administrator has decided to block X's IP address (and/or netrange).
Z is a forum website. X is spamming the forum. Z blocks X.
For this example, I will list three computers connected to the internet.
Normally, one would connect directly from X --> Z.
However, in some scenarios, it is better for the administrator of Z to restrict or disallow direct access and force visitors to go through Y first.
So, as before, we have data being retrieved by Y --> Z on behalf of X, which chains as follows: X --> Y --> Z.
What is different this time compared to a "forward proxy," is that this time the user X does not know he is accessing Z, because the user X only sees he is communicating with Y.
The server Z is invisible to clients and only the reverse proxy Y is visible externally. A reverse proxy requires no (proxy) configuration on the client side.
The client X thinks he is only communicating with Y (X --> Y), but the reality is that Y forwarding all communication (X --> Y --> Z again).
In the above scenarios, Z has the ability to choose Y.
A pair of simple definitions would be:
Forward Proxy: Acting on behalf of a requestor (or service consumer)
Reverse Proxy: Acting on behalf of service/content producer.
I found the diagram below to be very helpful. It just shows the architecture of a forward vs. reverse proxy setup from client to server over the Internet. This image will help you to understand qyb2zm302's answer and other answers better.
You can also watch this video from F5's DevCentral by Peter Silva.
Picture Source: Quora. However, as per Martijn Pieters, this image could be from Pulse Secure Community or Julien Pauli's site (in French) at developpez.com.
It reminded me of the classic proverb:
A picture is worth 1000 words.
Forward Proxy vs. Reverse Proxy (2012) explains the difference between forward and reverse proxies very clearly.
qyb2zm302's answer nicely details applications of proxies, but it slips up on the fundamental concept between forward and reverse proxies. For the reverse proxy, X → Y → Z, X knows about Y and not Z, rather than vice versa.
A proxy is simply a middleman for communication (requests + responses). Client <-> Proxy <-> Server
The proxy acts on behalf of the client. The client knows about all three machines involved in the chain. The server doesn't.
The proxy acts on behalf of the server. The client only knows about the proxy. The server knows the whole chain.
It seems to me that forward and reverse are simply confusing, perspective-dependent names for client and server proxy. I suggest abandoning the former for the latter, for explicit communication.
Of course, to further complicate the matter, not every machine is exclusively a client or a server. If there is an ambiguity in context, it's best to explicitly specify where the proxy lies, and the communications that it tunnels.
Some diagrams might help:
Forward proxy
Reverse proxy
The difference is primarily in deployment. Web forward and reverse proxies all have the same underlying features. They accept requests for HTTP requests in various formats and provide a response, usually by accessing the origin or contact server.
Fully featured servers usually have access control, caching, and some link-mapping features.
A forward proxy is a proxy that is accessed by configuring the client machine. The client needs protocol support for proxy features (redirection, proxy authentication, etc.). The proxy is transparent to the user experience, but not to the application.
A reverse proxy is a proxy that is deployed as a web server and behaves like a web server, with the exception that instead of locally composing the content from programs and disk, it forwards the request to an origin server. From the client perspective it is a web server, so the user experience is completely transparent.
In fact, a single proxy instance can run as a forward and reverse proxy at the same time for different client populations.
Proxy: It is making the request on behalf of the client. So, the server will return the response to the proxy, and the proxy will forward the response to the client. In fact, the server will never "learn" who the client was (the client's IP address); it will only know the proxy. However, the client definitely knows the server, since it essentially formats the HTTP request destined for the server, but it just hands it to the proxy.
Reverse Proxy: It is receiving the request on behalf of the server. It forwards the request to the server, receives the response and then returns the response to the client. In this case, the client will never "learn" who was the actual server (the server's IP address) (with some exceptions); it will only know the proxy. The server will or won't know the actual client, depending on the configurations of the reverse proxy.
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