What is Ping:  Ping is the total time it takes to send and receive a piece of information and is measured in milliseconds. Most people agree that a ping of 125ms is very playable for online first person shooters such as Battlefield 3 and Call of Duty: Modern Warfare 3. High pings are a double edged sword, it can both help and hinder you. On the positive side, a high stable ping can be compensated for, if you play a lot it will become second nature and won't affect your gaming experience negatively. On the flip side most high pings aren't stable, meaning its near impossible to compensate for, many servers kick for high ping, and you'll never be able to compete with the ultra low pings (40ms or less). So how can you find out what your ping is? Well most games will have it listed on the score board, if it doesn't you'll need to find the servers location or IP address.
 
Jitter: Jitter is the variation of ping over a given time period, the higher the jitter the more unstable your connection will seem. In video games, jitter is one of the worst problems you can face. Most connections will have a jitter of +/- 5ms meaning your ping will fluctuate by 5ms, for example a 45ms ping could be between 50ms and 40ms. The higher the fluctuation the more "lag" you experience, usually manifesting in "jumping" (see video).  Jitter is usually caused by other computers or programs, for example another computer on your network streaming hulu will cause ping spikes but it is also caused by unstable internet connection, overloaded nodes and such. This is because every few minutes hulu buffers, taking up more network resources and sending your ping higher.

Packet loss: Packet loss occurs when "packets" of data sent via a network fail to reach their destination. Here's an example, if my packet loss is 30% and I shoot 10 bullets at an another player,  only 7 rounds would be registered. That's an over simplification of packet loss but it's still a good example. Packet loss' effect on gaming is rather hard to explain, in some cases a packet loss of up to 15% can have zero effect on gaming, the problem is that packet loss is usually accompanied by other issues (high jitter and/or ping). Packet loss occurs at servers/hubs, usually were there is a lot of traffic and if you test your packet loss throughout the day you might notice an increase towards peak usage.