The client is the program used to play Ultima Online on your computer. There are currently 2 clients active.
On a slightly more technical level, a "client" is a program on your computer that is used to access information on another computer, called a server. Your web browser acts as a client, acquiring information from the servers hosting web sites. In this case, the UO client accesses information on UO's servers, called shards.
New UO players must download one of the current clients and install it on their computer. They must then run UO which will automatically check to see if it is the latest version and download Client Patches if necessary to update itself to the latest version. Sometimes, this initial patching can take a considerable amount of time, even on broadband connections.
These game clients are currently supported and frequently updated. They can be used to connect to official Ultima Online game servers. Both of UO's current clients also used the isometric perspective. Most other games similar to UO use a perspective that's closer to "first person."
The original and most popular game client. It has very low minimum specs and is highly optimized after over a decade of continual improvement. Unlike many similar games, UO's client used an "isometric" perspective, which basically seems to mean overhead, with a tilted camera. This was about the same perspective used by many of the Ultima single-player games upon which UO was based. It is still the most popular client to use playing UO and especially so among veteran players.
Latest Version CC: 18.104.22.168 - April 2, 2015
The current 3D-ish client, the Enhanced Client was developed from some of the underpinnings of the Kingdom Reborn Client but was purportedly highly optimized based on performance and player feedback. It was initially called the Stygian Abyss Client to coincide with the release of the Stygian Abyss expansion but as development of the two projects fell out of synch, it was increasingly often referred to as the Enhanced Client. While in a high state of stability and much better performing than the Kingdom Reborn client, it is still under active development but does enjoy a modest adoption rate, especially among new players.
Latest Version EC: 22.214.171.124 - April 2, 2015
These game clients have been retired and can no longer be used to connect to official Ultima Online game servers.
Third Dawn Client
Introduced with the Third Dawn expansion, the Third Dawn client was created with 3D technology but kept the isometric perspective. It was the first attempt to replace the 2D client and initially, for a time, only players using this client could enter Ilshenar.
The Third Dawn client never caught on and was discontinued when the Kingdom Reborn(or "KR") client was released.
Final Version: 126.96.36.199 - March 28, 2007
Kingdom Reborn Client
The graphical base was, more or less, 3D-ish. The user interface was very different from either the original 2D client or the Third Dawn client, and was closer to more modern online games.
EA once stated that the original 2D client could be phased out if 80% to 90% of the player base was consistently logging on using the Kingdom Reborn ("KR") client alone but adoption was never widespread and it was discontinued in favor the Enhanced Client(also called Stygian Abyss(or "SA") client.
Final Version: 188.8.131.52 - July 14, 2009