Yang style tai chi is the first style of tai chi to be properly formulated in to forms and one of the most popular styles today. In my opinion it is the most true to the principals of Tai Chi, being that even today after hundreds of years has changed very little, (bar the new short forms) There are those that say they are doing Yang style tai chi, but if it is not true to one of the foundation forms, such as the Yang Lu Chan form or the Yang Cheng fu form, then it is probably not true to Yang style. The newer short forms try their best to look like Tai Chi but in my opinion are just a shell and underneath there is no essence left.
Yang Lu Chan 1799-1872, was the inventor of the old yang style. Old because it is the oldest of the Yang style forms, many regard this as the ultimate Tai Chi form, as it has a perfect balance of slow energy gathering movements and explosive energy releasing movements. It also has a perfect balance of movements on both left and right sides. Some other forms do not have such a perfect balance and have to be done on both left and right sides to get what you would get from the Yang Lu Chan form on one side alone. It is the epitome of what Tai Chi is, a martial/healing art. Unlike other forms which are just for health, the old form also teaches you about deadly Dim-Mak fighting. But still retains the highest healing qualities of all the forms.
Yang Cheng fu 1883-1936, was the inventor of the Yang Chen Fu form, which is also know as the long form because most schools don't teach the Old form so this has become to them the longest form. Yang Cheng Fu changed the old form slightly, taking out the more explosive moves and some of the more strenuous jumps and kicks. This enabled everyone to practice tai chi, including older more frail students. Even though he changed some of these moves, he did it in a way as to not disturb the energy flow and original intent of the form enabling everyone to get the great health benefits of tai chi. After he invented the Yang Cheng fu form he said...
"to change the form any further, would be to ruin what Yang Lu Chan had taken so long to create"
The reason for this is that if too many changes are made to the original Yang Style Form then the energy flows are changed too. If you perform movements that go against the body's natural energy flow, you will be doing your health a great damage, by causing blockages to your Qi. This is much more important when getting to the higher levels of tai chi, so if you intend on making tai chi a part of your life and progressing to the higher levels, take care to make sure you are practising one of the two original Yang Style forms.