Anybody that can help me understand some mortgage stuff - HELP!

[Travel Guru]

Okay,I am soooo not mortgage savvy......Our current mortgage is through the State Credit Union (since I work for the state as a teacher). We have a first time home buyer mortgage which was a 2 year ARM. It will be 2 years come November and we want to refinance. We were going to go to a fixed, but the interest rate is 1% more than it is right now and our monthly payment will go up almost $100. That may not sound like much but it kind of stresses me out. I was looking at the ARMs offered by the credit union and don't really understand why we wouldn't think about doing this again. We could do another two year ARM at 5.25%.... but it's all this margin, APR, etc. that I don't understand. And they add .75% of your remaining principal to the balance loan.... Can somebody explain all this to me? What is the margin and how is it decided? On the credit union page they have a chart that lists the ARM, the margin, and the new rate. Is that the final rate? Here's the link:Adjustable Rate Mortgages (ARM) (http://www.ncsecu.org/Loans/AdjustableMortgage.html)I just get all confused and want us to do what's best/smartest for us, but also want to help us save as much as $$ as possible right now in this tight economy.