Almost all bikes since the early 80s have had what is usually called "crossover" gearing. The basic idea is that you have one range of rear cogs for each chainring, so that you only need to shift the front once, usually accompanied by a 1 or 2 cog rear shift in the opposite direction.
so, using a 9 speed road bike, with 53/39 chainrings and a 12-27 cassette, starting from the lowest gear, the pattern is something like this:
(numbers are cogs, starting from the largest)
1->2->3->4->5->6
____________________<Front Shift>
_____________ ______________________4->5->6->7->8->9
So the crossover shift requires a double shift, up one in the front and down two in the back, to get the next gear in the progression. In reality, you'll likely only do this on the flats, especially if it's windy. Cresting a climb, especially in rolling hills, mostly it's just the front shift as you coast up to descending speed.
I find
this graphical gear calculator to be really helpful in seeing what the gear intervals and shift patterns look like.
Sheldon's gear calculator gives you the actual numbers and percentages, (cooked three different ways and served with or without fries,) and has the added bonus of letting you print out a properly-sized gear chart that you can tape to your stem.
Both are great, and should be used, played with, and pondered until Enlightenment is attained.
--Shannon