The International Commission of Stratigraphy has published an updated chronostratigraphic chart.
You can download a high resolution pdf as well as a jpg version.
This is a good resource if you want to know the time spans for all those hard to pronounce units that geologists keep referring to in their talks and articles.
The columns in the figure are hierarchical divisions of geologic time based on the rock record. Take for example the columns on the extreme right. Proterozoic Eon is an interval of time defined by rocks that formed between 2.5 billion years ago and ~539 million years ago. The Neoproterozoic Era is a subdivision defined by rocks formed between 1 billion years ago and ~539 million years ago. And the Ediacaran Period is a time unit represented by rocks formed between 635 million and ~539 million years ago. A significant change occurred around 539 million years ago. Shelly fossils began appearing in the rock record. This marks the end of the Proterozoic and the beginning of the Cambrian Period.
Notice something interesting?
Take a look at the internal divisions of the Archean and the Proterozoic Eons. The Archean is divided into fairly uniform 400-300 million years units, while most of the Proterozoic is divided into roughly 200 million years units.
In contrast, the Phanerozoic Eon subdivisions have unequal age spans. By around 550 million years ago, unicellular and multicellular creatures evolved the ability to build hard skeletons made up of calcium carbonate, calcium phosphate, and silica. This resulted in better preservation of their remains in the rock, giving geologists the ability to track evolutionary changes. The boundary between different subdivisions you see on that chart is a recognition of the appearance of a new species or species assemblage. Evolution's clock has an irregular beat, making for unequal chrono units.
More uncommonly, boundaries are defined by chemical shifts that mark a change in the earth's environment. For example, the newest subdivision of the Quaternary Period, the Meghalaya Age, is recognized by shifts in the oxygen isotope composition of stalactites from a cave in Meghalaya, signalling global drought events beginning 4.2 thousand years ago.
The Precambrian was a microbial world. Microbes too were undergoing continual evolutionary changes, but they did not build hard skeletons and their tiny soft remains don't preserve well at all. Geologists can't use microbial evolution to subdivide the Archean and the Proterozoic. There are no systematic temporal changes in rock types as well.
The boundary between the Archean and the Proterozoic does represent a significant transition in earth processes. It marks a shift from a hotter and more tectonically active Archean to a cooler earth, where large rafts of continents had formed. Many sedimentary basins that developed on this continental crust did not undergo the intense deformation and metamorphism observed in Archean sedimentary and volcanic rock associations. But, like the Archean, through most of the Proterozoic too, there is no uniquely changing fossil record to act as anchors for subdivision.
By around 700 million years ago, rapid global changes in climate and the evolution of larger organisms has allowed for a finer resolution of the rock record. Both the Cryogenian, which preserves signals of widespread glaciations, and the Ediacaran, with a better fossil record of larger multicellular creatures, are smaller subdivisions of the Proterozoic.
Through my geology career, I have been lucky to have worked on rocks from across the time scale. My M.Sc. thesis was on Orosirian, Statherian and Tonian age rocks from the Proterozoic. My Ph.D research was on Katian and Hirnantian age rocks from the Ordovician Period. And I have done smaller projects on the Oxfordian of Jurassic, and some imprecisely dated rocks from the Eocene Period.
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