Carrier
If you are a non-corporate BlackBerry user then your BlackBerry is assigned to a BlackBerry Internet Server (BIS) that is installed either on your carriers network or Research In Motion's (RIM) network. The BIS is a slightly different version of the BES that allows only email and web browsing.

Your Home (Unite)
If you want to have your own BES at home, or if you run a small business, you can download the free Unite software from RIM. Unite is essentially a stripped down BES that runs on a PC in your house. It supports up to five BlackBerrys.

So you are probably thinking that if your BlackBerry can be assigned to a BES, a BIS, or Unite that you would have three browsers. You would be correct, but your carrier can add another one, and if your BlackBerry has Wi-Fi, you will have another.

In total, five possible browsers. Here they are:

1. BlackBerry Browser

1. This browser is only used in a corporate environment. All web browsing goes via the BES using the MDS-CS service. The MDS-CS service acts as a proxy for the BlackBerry Browser and also strips out web page content that is not suitable for the BlackBerry.
2. MDS-CS also compresses the data before it is sent to minimize wireless data usage and resizes images based on the device they are going to (yes it keeps track of the screen resolution of each device registered with it and resizes images accordingly).
3. Because the BlackBerry Browser gets all of its content via the BES's MDS-CS service, the BlackBerry user can browse internal company web sites. If the network administrator wants to block certain web sites on a desktop and BlackBerry, they simply configure the BES to use the company's internet proxy serves and/or firewall.
4. The browsing speed is affected by how busy the BES is. The BES prioritizes email over web browsing and other tasks, so if your company's BES is too busy, your web browsing will suffer.
5. Data path: BlackBerry <-> Carrier <-> RIM NOC <-> BES <-> Internet

2. Internet Browser

1. This browser is on every BlackBerry unless your BlackBerry is on a BES, and the administrator has explicitly disabled it. All web browsing from this browser goes via the BIS using the MDS-CS service.
2. Since web browsing is going via MDS-CS, all of the same features that are listed above apply.
3. It is certainly conceivable that a busy BIS can affect web browsing.
4. Data path: BlackBerry <-> Carrier <-> RIM NOC <-> BIS <-> Internet

3. WAP Browser

1. WAP stands for Wireless Access Protocol and is an old wireless standard for web browsing (when mobile browsers were very, very limited). The carrier has a WAP Gateway that this browser connects to. The WAP Gateway is setup to get a very limited subset of web sites.
2. The WAP gateway translates regular web pages to a language that the WAP browser can understand.
3. Data path: BlackBerry <-> Carrier <-> WAP Gateway <-> Internet

4. Unite Browser

1. This browser is available if your BlackBerry is assigned to your Unite server. Since Unite is a stripped down BES, it too has an MDS-CS service and so all of the same MDS-CS features are available as listed above
2. Your PC running Unite must be running at all times otherwise this browser cannot function.
3. Data path: BlackBerry <-> Carrier <-> RIM NOC <-> Unite <-> Internet

5. Wi-Fi Browser (sometimes called the HotSpot Browser)

1. This browser is available if your BlackBerry has Wi-Fi (BlackBerry Bold, Curve 8320, etc.).
2. This browser does not use any servers but goes directly to the internet.
3. This is by far the fastest browsing experience since it is just you and the internet
4. Data path: BlackBerry <-> Wi-Fi HotSpot <-> Internet
   

Please note that if your BlackBerry is assigned to a BES, your administrator can remove all browsers, or leave you with just the BlackBerry Browser. This is normally done for compliance and/or security reasons. (Source: Craig Johnson)