As per my comment I am not certain what you are looking for, but when I am fitting time series after a bit of a hiatus from them I tend to grab my copy of Time Series Analysis and Its Applications for more theory questions and I look at a few different sites online (also do some googling to see if there are any sweet new ones):
The CRAN taskview on time series gives you a good look at just how many things you can do
Is a nice walk through of some time series analysis in R. I personally do much of my statistical learning through example (which generally means following guides like this in R), so this guide is a favorite of mine.
This link is a decent look at ARIMA outside of R, it walks you through what different models mean.
Finally, you can always check out wikipedia if you are just looking for statements for formulas. These are just the ones I have book marked, so maybe some other folks will contribute their favorites. As I said in comments, if you expand on what you are looking for more specifically you can probably get better links from me or one of the folks that follow time series closer than I do.