A number of grass families, the bents, the fescues, the meadow-grasses and others have this familiar 'Christmas tree' flower head or florescence. Close examination of the structure of the flowers, the leaves and other component parts will lead you to the species but for many casual observers, and I include myself in this, distinguishing between grass species can be quite daunting sometimes.
It is here that bringing in other factors that one can start to at least narrow down the choice and so it is with wood millet (Milium effusum). First and foremost this is a species of broadleaved woodlands, especially areas within the wood that are damp, and few similar species are likely to be found in this environment. Next, it is a very tall grass growing to around four feet and that, too, is uncommon for species with these types of flower heads. Finally, the flowers are actually quite distinctive with the branches some way apart and have pale green spikelets that seem to shimmer in the breeze.
There is still room for error even taking these factors into account and botanists will cringe at my generalisation I am sure but if you are in the south of England (specially in Dorset like me) in woodland on chalky but heavy, damp soil and you see a tall grass like this then it is almost certainly going to be wood millet.