Strontium is the 38th element, is the fourth of the alkaline earth elements, existing below Calcium on the periodic table. Strontium is present in about 400 parts per million in the earth's crust. This means it is neither rare, nor common. However, ores that have strontium in a concentrated form are somewhat more rare.
Strontium's chemical properties are in line with being an alkaline earth metal. As they increase in atomic number, the elements on the left hand of the periodic table, the alkaline and the alkaline earths, become more reactive. Strontium has two electrons to give, and oxygen has two electrons to take, so free strontium would oxidize rapidly in air. In other words, it would burn. Strontium compounds are always found in the oxidized form, and pure strontium must be kept in an inert environment or it will begin to burn. Other than being more reactive, strontium's chemistry is almost identical to that of calcium.
Strontium has few economic uses, and is mostly used for glass in Cathode Ray Tubes (a rapidly declining industry), and for producing bright colors in flares and as an alloying agent with iron in magnets. In 2009, the United States consumed about 10,000 tons of strontium. Compared to the 100 million metric tons of iron products that the United States produces, or even the 450,000 tons of a niche metal like chromium, the industrial use of strontium is rather minor. If there was no strontium available for industry, it would probably have a very small impact on our lives.
Strontium's biological role is much like its industrial role. All living things contain some strontium, and for most purposes, a living creature will process and metabolize it the same way that they produce calcium. The two main ways that an animal uses calcium is as a cation and as a structural material, in either bone or shell. Strontium should be mostly interchangeable with calcium for these uses. In fact, there is some medical thought that supplements of strontium can help fight osteoporosis. However, strontium is probably not totally interchangeable with calcium, since an animal who had no calcium could probably not make bones only out of strontium.
So, to sum up: in its technological and biological uses, strontium is widespread, yet not very important.
Which brings us to one of the negative aspects of strontium, which is almost incidental to its chemical properties. A certain isotope of strontium, Strontium-90, is present as a daughter isotope in nuclear fission. When a uranium atom splits, it can form a number of different smaller isotopes. One of these is strontium-90. Strontium-90 is itself radioactive, having a half-life of 30 years, before it emits a beta particle and decays into Yttrium-90, with a half life of a few days, after which it emits a beta particle to turn into the stable Zirconium-90. As far as radioactive decays go, this is a rather simple and not-too-energetic decay chain. However, for a few reasons, strontium-90 is one of the most dangerous radioactive isotopes that is created. First, strontium concentrates up the food chain. Dilute strontium in the atmosphere will fall on grass, where it will be absorbed and then concentrated, to be further concentrated when cows eat it, which then concentrate it in milk, which is then drunk by people who concentrate it in their bones. Secondly, strontium is stored in the body in a very vital place: since it substitutes for calcium, it will be in the bones, where its radiation will affect the bone marrow. Third, its half-life of 30 years means that once it has entered the environment, it will be present in sizable quantities for a century or two. And yet its half-life is still short enough that it is an energetic emitter. And thirdly, there is no regular mechanism that animals have for eliminating strontium from their bodies. Some radioactive substances can pass through the digestive system without being absorbed. Others will be absorbed but then excreted. But strontium-90 will enter the body and then stay there.
The only good news about strontium is that since it has a very high heat of vaporization, it is not likely to escape a nuclear power plant during a garden-variety accident, although an explosive accident like Chernobyl did release large amounts into the atmosphere. It can also be released during atmospheric tests of nuclear weapons, although such things are thankfully rare.
So this then is the double story of strontium-- an ubiquitous, mostly harmless metal that has a radioactive isotope that could be the primary killer if the world ever goes to nuclear war.