Recent publicity over a new fossil, Darwinius masillae, effectively touted it as a missing link.
Is there a link that is missing, just waiting to be found? Has it not been found already? Why do 'they' keep talking about it as missing?
The term is not precise. In general, people have used it to refer to an evolutionary link between humans and.. well apes, monkeys, mammals, or the rest of the evolutionary tree in some way.
It's effectively a lay term for a transitional form between one species/grouping and another. In the sense that there are an awful lot of transition to go through, and the fossils found so far represent only a fraction of them (Tudge, 2000, p462 estimates that there were several thousand primate species, of which only about 250 have been found). Yet the reality of evolution is never in dispute, as the evidence is already overwhelmingly incontrovertible.
Evolutionary progress is best defined in clades, complete groupings of all species originating from a given ancestor.
Modern humans (Homo sapiens sapiens) emerged as a distinct species very recently - about 100,000 years ago. We're sufficiently close to neanderthals (Homo sapiens sapiens, extinct about 30,000 years ago) that we have some neanderthal DNA.
There were a number of other Hominid species since they first appeared about 4.4 mya (million years ago). Humans are grouped with their closest relatives, the great apes, as Hominoids, first dating from about 14mya. Anthropoids (aka simians), the clade that then includes monkeys dates back about 34 million years. Primates, the clade that then includes lemurs, tarsiers and more, dates back to about 55mya, going by the fossil record. Yet other analysis (see Tudge, p463) concludes that primates must have originated around 70mya, which puts them past the KT (meteor) threshold, to the time of dinosaurs.
All this places modern humans as a particularly recent species. Many transition fossils have been found to trace a steady evolutionary trail.
The new fossil, Darwinius masillae, dates back 47 million years. It is prosimian - ie, a primate that is not an Anthropoid [prosimian is descriptive, but not a clade, since it doesn't include an ancestor and all descendant species]. This fossil is extraordinarily complete, due to the particular circumstances of death and preservation - and that is perhaps its most remarkable feature. Anything more is just publicity - paleontologists are as prone to publicity as any profession. While it contains features features of some simians, and lacks some features of some prosimians, whether it is a direct ancestor of humans is definitely not yet resolved. It could have branched off the primate line before it led to the simian Anthropods, as did other prosimians.
22-May-09 postscript: In passing, I heard some comments on a radio programme that may speak more directly to this fossil's significance. First, that it may fill in a gap in the direct human lineage (if it is verified to be situated directly in line) - and that's always helpful. The bloke (whose name I missed) also suggested there were aspects of the growth detail of this fossil - being as finely preserved as it was - that are particularly useful in a specimen this old.
[Ironically, my ears first tuned into the words on the radio when I heard the phrase 'missing link' - which characterisation that bloke, too, was hosing down.]
Useful references include:
Dawkins, B (2004): An Ancestor's Tale. Phoenix, London.
Tudge, C (2000): The Variety of Life. Oxford University Press, Oxford.
Wikipedia entries on simian, prosimian, Darwinius masillae, Homonid, etc.