The Earth is pretty old. Before we start talking about rocks, it is important to understand how to talk about time from scales of a human lifetime (50-100 years) to the age of the planet (~4.6 billion years). This is geochronology [Etymology: geo: Earth; chrono: time; logy: study].
Earth time is measured in units of a year, or annum (symbol: a). To this can be added metric prefixes, such as kilo (thousand), mega (million), and giga (billion). So something happening 100,000 years ago would be something which happened at 100 ka; an event 66 million years ago would be at 66 Ma, and something 2 billion years ago would be at 2 Ga. Durations of time use the same units, and it should be clear from context whether the time in question is a duration or a relative time before present.
At the top of this post is the most recently published geologic timescale. This timescale represents the best science available at the time of publication. However, as more evidence is gathered, these boundaries can be shifted around. To most people, the shift in the Cretaceous-Paleogene boundary (the break between the left-most major column and the second) from 65.5 Ma to 66.0 Ma in the newest revision of the timescale doesn’t mean much. However, if you are trying to understand causes and effects near that time period, it can make a big difference!
One major feature to notice about the geologic timescale is that it is depicted in a non-linear fashion. That is, the right-most column (Precambrian) represents 88% of Earth’s history. The Paleozoic represents 6%, and is longer than the Mesozoic and Cenozoic combined. The Cenozoic contains only the last 66.0 Ma, or 1.4% of Earth’s history.
Our ability to tell time for more recent events is better than for older ones: good geochronology has uncertainties between 1% and 0.1% of the age. For something 66 million years old, the uncertainty in age can be around 66 ka or less ; for something 2 billion years old, the uncertainty is likely to be closer to 2 million years. A lot can happen in 2 million years!
Getting back to the timescale chart, present-day is at the top left. As you go down the column, you are going back in time. The bottom of the column connects to the top of the column to the right of it. At the bottom of the right-most column is the earliest part of the rock record on Earth. Very little remains of this age which has not been recycled into new rock.
The divisions are made on the basis of major events in Earth history (or on big round numbers for the oldest parts). Many divisions represent shifts in the types of life (often plants or animals) found on Earth. For instance, the base of the Mesozoic is defined by the first appearance of Hindeodus parvus (a conodont, which is kind of like a modern eel) in the fossil record. The defining species tend to be marine, because marine records are usually more continuous, are more likely to be preserved, and have more species found across the globe (technical term: cosmopolitan).
Several different methods exist for determining the age of a rock: radio-isotopic dating, biostratigraphy, and paleomagnetism being the major methods employed. For sedimentary rocks younger than 50 Ma, astrochronology (or orbital tuning) can be used to match climate signals found in the rock to the Earth’s tilt, precession, and eccentricity.
Radio-isotopic dating is the most reliable method.* By measuring how much radioactive decay has happened since a rock formed, and knowing the rate at which the decay occurs, the date of formation can be determined. For volcanic ash deposited in a sedimentary sequence, that formation is roughly equivalent to the eruption date.
Biostratigraphy is used when fossils are present but no suitable material for radio-isotopic dating is available. The appearance and disappearance of various species (again, usually marine species found world-wide) are used to mark time. This is relative, and subject to sampling and preservation biases, but can yield rough answers in places where otherwise there would be no answer at all. However, it is difficult to apply in the Precambrian, because there were fewer, less-complex lifeforms at that time. Most of the diversity of life on Earth post-dates the Cambrian explosion at ~541 Ma. For rocks newer than that, the fossils found within them can give a rough estimate of the age.
Paleomagnetism is used when there are recorded direction changes in the Earth’s magnetic field within a section of rock, and at least some of that rock is fairly precisely dated. By counting magnetic field reversals from a known-age point, the ages of the section in question can be determined by correlation to a well-dated section with the same reversals.
In summary, the Earth is old, and the ages of events are generally known to 0.1-1% precision. The geologic timescale is broken up into smaller parts, and the size of divisions generally decreases as modern time is approached. Units of Ma (mega-annum, million years) are appropriate for much of Earth’s history, with ka used for recent events and Ga used for the oldest parts. As we continue on our exploration of Heard Island and other topics, the geologic timescale will be a very useful tool for understanding how pieces fit together in time.
 Gradstein, F.M, Ogg, J.G., Schmitz, M.D., et al., 2012, The Geologic Time Scale 2012: Boston, USA, Elsevier, doi: 10.1016/B978-0-444-59425-9.00004-4.
 Renne, P. R., Deino, A. L., Hilgen, F. J., Kuiper, K. F., Mark, D. F., Mitchell, W. S., III, Morgan, L. E., Mundil, R., Smit, J. (2013) Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science 339: 684-687, doi: 10.1126/science.1230492.
 Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R. and Wijbrans, J.R. (2008) Synchronizing the 40Ar/39Ar and astronomical clocks of Earth history. Science 320: 500-504, doi: 10.1126/science.1154339.
* Its theory and operation is more complicated than this blog post can contain. I may try to write one later which has much more of the nuance and technical details.