Layered Rocks Rule!
A mitten butte in Arizona's Monument Valley - made famous by Wile E. Coyote and the Roadrunner.
A geologist doesn't usually have the luxury of knowing how old layered rocks are, a number earth scientists call the "absolute age." Instead, earth scientists are forced to use whatever clues they have to determine a layer's "relative age." Igneous rocks like lava flows and volcanic ash are an exception to the rule, because scientists can use radiometric dating to pin down the absolute age. The presence of a layer with a known absolute age lets geologists use established rules to puzzle out the age of a specific sedimentary layer compared to other layers and geologic features in the immediate neighborhood, letting them say that the layer's older than this rock layer and younger than that one. 
More Layers... Sigh...
Types of Rocks
Most layered rocks are bedded sedimentary strata, the kind of rock that forms at the earth's surface from compressed, eroded bits of older ones.  Because these beds are composed of billions of tiny pieces of many different older rocks, it is hard (if not impossible) to establish the absolute age of sedimentary strata. Bracketing the absolute age of any one of these layers in a range of ages is more likely, but being able to do that depends on what else can be found nearby. Adjacent layers of igneous rocks, like lava flows and volcanic ash, are the best candidates for measuring absolute age usingthe techniques of radiometric dating.  Ash layers are a special case, because they contain datable igneous minerals and they have settled on the earth's surface like a blanket, as if they were sedimentary. 
The Basis of Radiometric Dating
A pictorial explanation of the half-life of an unstable uranium isotope that decays to lead.
Radiometric dating operates on the principle that isotopes of certain elements are unstable. These "parent" atoms will eventually decay spontaneously, turning themselves into atoms of a different element called the "daughter." The key to radiometric dating is that this conversion process occurs at a predictable rate, called the half-life. In other words, if you compare the number of atoms of the parent isotope in a sample to the number of atoms of the daughter isotope present, you can use a formula to calculate how long ago decay started - and that's the absolute age of a mineral. You can't just spread all those atoms out on a piece of paper to separate them like coins or marbles, though; you have to use very high-tech equipment. Performing radiometric dating, in fact, requires a well-equipped laboratory that includes a particle accelerator.
Scientists often refer to the several different radiometric dating techniques using the names of the elements involved, parent to daughter. That gives rise to methods called uranium-lead (U-Pb), potassium-argon (K-Ar) and rubidium-strontium (Rb-Sr), among others.  Although the process of radiocarbon dating may be more familiar to most people than those methods, it is rarely useful in dating sedimentary strata because the maximum age range of the method is only about 50,000 years. That might seem like a long time to most, but geologists tend to think of time in terms of millions to tens of millions of years.
Superposition, In Pictures
Law of Superposition
Geologists invoke a simple, long-established rule to compare the relative ages of any two or more sedimentaryy strata. This is Steno's Law of Superposition, which states that in any stack of layered rocks, the oldest ones are always found at the bottom and the younger ones are found at the top. In the case of our unknown layer that's mixed in with beds of volcanic ash, the Law of Superposition tells us that it is older than an ash bed above it and younger than an ash layer below it. 
How Old Are You, Anyway?
Estimating Absolute Age
With all those puzzle pieces assembled, a geologist can work on estimating the absolute age of a sedimentary bed between two volcanic ash beds. First, radiometric dating determines an absolute age for the ash above our subject rock layer (for example, 100 million years old) and a second absolute age for the ash bed below the subject layer (let's say it's 110 million years old). The Law of Superposition says any sedimentary rock that's between those two ash horizons must be younger than the one below it and older than the one above it. In our example, this means the layer we've been studying has an absolute age between 100 and 110 million years old. 
The original of this article was rejected by one of the science section content editors at eHow who was confused about what a sedimentary rock is. So much for that BA in journalism...