The Common Swift (Apus apus) is a member of the family Apodidae (typical Swifts). The Apodidae are in the order Apodiformes together with the family
Hemiprocnidae (Treeswifts, see for example this awesome video!). Swifts and Treeswifts may look superficially similar to swallows, but those are in a completely different family (Hirundinidae
(swallows)) and even in a completely different order (Passeriformes (songbirds)). Swifts and Treeswifts are in fact more closely related to Hermits and
Hummingbirds (the family Trochilidae). The orders in which the Swifts, Treeswifts, Hermits and Hummingsbirds are placed are all placed in the same
superorder; Apodimorphae. Apodimorphae, Apodidae and Apus are all derived from "no feet" in latin. Special birds though they all may be, they are not
legless, the legs are small and of very little use beside perching. Common Swifts however cannot perch on twigs and branches due to the build of
their feet, those are specially adapted for hanging on vertical surfaces.
  
Common Swift's feet are designed for hanging on vertical surfaces for good reason. Orginally, Common Swifts were birds that nested in rocky areas. Over time,
they've replaced natural rocks with man-made building, something other species of birds have done as well (like for example the Red Blackstart,
Phoenicurus ochruros).
But even though Common Swift's feet can grip vertical surfaces very powerfully, they don't spend a whole lot of time doing so. Common Swifts are birds that
'belong' in the sky. They have long wings with strong wing muscles, which make them excellent flyers. Their bill is short but their gape is very broad, which
helps in their hunt for flying insects. Common Swifts hunt for flying insects very efficiently, during their stay in Europe, a single family eats
hundredthousands of insects. The fact Common Swifts and Swallows have a fairly similar diet is the cause for their similar appearance, all are
adapted to catch flying insects. The evolution of the Common Swift however has enabled it to do much more than just eating on the wing. Common Swifts can
also spend the night on the wing and even mate flying! When nesting they will spend the night in the nestsite though, and mating can take place in the
nestsite as well, possibly depending on the weatherconditions which location is chosen. Swifts that don't have a nestsite yet have little choice but to
spend the night in the sky, along with fellow homeless Swifts. Sometimes these homeless Swifts spend the night by hanging among the foliage in trees, of
which an excellent video can be found here on YouTube (along with many other nice
Swift videos.
The places Common Swifts chose to nest in are very diverse. Nesting can take place in all kinds of hollow spaces, under rooftiles, in cracks or other
openings in walls and in atticks are just a few examples. Common Swifts are very dedicated to their nestsite, returning to the exact some cracks or hole
every year. A lot of the kind of places that Swifts use to nest are the result of imperfect human building or old age of the buildings. When building are
renovated or torn down to be replaced by new buildings, little to no room is left for these beautiful and special birds. Helping Swifts can be very simple
in some cases, take a look at what you might be able to do in the Swift help section.
Try some of the links in the links-section for more info about Swifts.