The head is an inflorescence with numerous small flowers crowded on an expanded structure, called 'receptacle', that is often surrounded by an involucre of small bracts or scales. Heads are often confused with single flowers, e.g. in the case of daisies, where the yellow center of the head is occupied by small regular (radially symmetrical) flowers, whereas the margins have white, irregular (bilaterally symmetrical) flowers which resemble petals:

In the following there is a gallery of different types of heads:
Heads of Knautia and Scabiosa :

Heads of Achillea:

Heads of thistles:

Heads of Artemisia and Matricaria discoidea:

Heads of Erigeron (Conyza):

Heads of Aster and Erigeron :

Heads of Phyteuma:

Heads of Globularia and Jasione:

Heads of Asteraceae of the Liguliflorae subfamily (such as Taraxacum):

With a little practice, it is easy to understand whether the flowers are arranged in heads or not. However, in some cases the heads host a few flowers only, and then one has to carefully observe the heads to understand that the apparent 'petals' are actually 'flowers'. Examples are the heads of the genus Adenostyles...:

...those of the genus Solidago...:

...those of the genus Ambrosia..:

and those of Lactuca muralis which bear only 5 strap-shaped flowers that very much resemble the petals of a single flower:

Flowers not in heads Attention! This option includes also species with flowers arranged in inflorescences similar to heads, but not surrounded by an involucre of scales and bracts, such as those of the genus Trifolium:

Furthermore, contracted umbels such as those of some species of Allium should not be confused with heads (the flowers are stalked, not sessile!):

In the genus Eryngium (Apiaceae) the flowers are arranged in strongly contracted umbels which very much resemble the true heads of the family Asteraceae, because they are surrounded by an involucre of bracts: they can be reached selecting both the option 'flowers in heads' and the option 'flowers not in heads':