The speculation regarding what these ratings mean has been very interesting and some of the guesses as to what the numbers actually refer to have been pretty imaginative. Some have speculated that 'Home' referred to your personal latency and 'World' was Blizzard's latency. This is incorrect.
In essence, 'Home' refers to your connection to your realm server. This connection sends chat data, auction house stuff, guild chat and info, some addon data, and various other data. It is a pretty slim connection in terms of bandwidth requirements.
'World' is a reference to the connection to our servers that transmits all the other data... combat, data from the people around you (specs, gear, enchants, etc.), NPCs, mobs, casting, professions, etc. Going into a highly populated zone (like a capital city) will drastically increase the amount of data being sent over this connection and will raise the reported latency.
Prior to 4.0.6, the in-game latency monitor only showed 'World' latency, which caused a lot of confusion for people who had no lag while chatting, but couldn't cast or interact with NPCs and ended up getting kicked offline. We hoped that including the latency meters for both connections would assist in clarifying this for everyone.
As is probably obvious based upon this information, the two connections are not used equally. There is a much larger amount of data being sent over the World connection, which is a good reason you may see disparities between the two times. If there is a large chunk of data 'queued' up on the server and waiting to be sent to your client, that 'ping' to the server is going to have to wait its turn in line, and the actual number returned will be much higher than the 'Home' connection.
"Well, great," you may say, "but what does that mean to me?!"
Not much, maybe, but I wanted to focus on how local (or network) factors can (and will!) affect these numbers.
Here are the most common causes of high pings/latency (on both Home and World):
2) Packet loss
4) Addons (yes, those wonderful UI modifications)
5) Firewalls (some firewalls do interesting things to latency... try playing without it to see if it helps!)
5) Mis-configured or defective home routers (please temporarily bypass before anything else)
6) Quality of Service and Traffic Management Systems performing packet queuing of some sort.
7) Net link saturation (not necessarily your ISP, but somewhere between you and Blizzard)
Lowering video settings (especially view distance) has the added benefit of lowering the amount of data your connection is asked to convey, as well, so even that can be a valid troubleshooting step.
If your 'Home' connection latency is low and your 'World' connection latency is high, that frequently indicates that there is some sort of QoS congestion controls being applied to your internet connection, at either the micro (LAN) or macro (WAN) level. A common symptom would be that you would be able to chat, but not to cast.
If both connections report high latency, that means your connection to our servers, in general, is almost completely saturated, or 'overflowing'. Without making any claims where that saturation lies, that seems to have been the most common case to date.
Please refer to our support pages (such as battle.net/support/article/network-card-support) or contact a technical support representative directly for further information and troubleshooting.