Starting with the basics and keeping it simple, Solid Edge will automatically create a varible for every dimension. Like fwc stated above, you will have to set name by to both in order to see these varibles. You can change these varibles (rename and add formulas) in the varible table or by double clicking on the dimension.
Taking it a step further, you can add a varible that doesn't already exist such as "L" with a value of "12". You can then change an existing varible such as "L1" to a formula of "L/3". This will give you a value of 4 and the varible will gray out because it will always be controlled by the varible "L".
Advanced, you can link varibles between documents. Example being, the length of "this part" will always equal 1/2 the length of "that part".
Using varibles is very powerful and easy once you understand what's going on. It's also a good idea to add a icon to your toolbar so you can make quick changes to a part rather than dynamic edit, especially if you are doing any patterning.
In the assembly environment, if you haven't checked out peer varibles, do so. This is another way of making alot of changes at once. Be sure to update your links and relationships once you make the change though. Also, make sure your parts that you want to change are activated.
I hope this helps. If not, there's alot of info on this in the help section of Solid Edge.