The galah is about 35 cm (14 in) in length, and weighs 270–350 g (10–12 oz). It has a pale silver to grey back, a pale grey rump, a pink face and breast, and a light pink mobile crest. It has a bone-coloured beak, and the bare skin of the eye ring is carunculated. It has grey legs. The sexes appear similar; however, adult birds differ in the colour of the irises; the male has very dark brown (almost black) irises and the female has mid-brown or red irises. Adults are more brightly coloured than juveniles. Juveniles have a greyish breast, crown, and crest, and brown irises with whitish non-carunculated eye rings.
• male
• Female (note the reddish iris)
• A juvenile in Sydney
• female in flight
• male in flight

The galah can be found throughout Australia, and is absent only from the driest areas and the far north of Cape York Peninsula. The galah has been introduced to Tasmania through anthropogenic means and there were no sightings prior to 1848. A large population expansion occurred in the 1960s following many escapees from captivity. It is common in metropolitan areas such as Adelaide, Perth, and Melbourne, and abundant in open habitats that offer at least some scattered trees for shelter. It is common in all habitats in its range except for dense forests, especially those with high rainfall.

While it is mostly found in inland areas, the galah is rapidly colonising coastal regions. The changes brought by European settlement, which have been disastrous for many species, have been highly beneficial for the galah, because of the clearing of forests in fertile areas and the provision of stock-watering points in arid zones.

The galah has been introduced to New Zealand.

The classification of the galah was difficult. It was separated in the monotypic genus Eolophus, but the further relationships were not clear. Obvious morphological similarities are shared between the galah and the white cockatoos that make up the genus Cacatua and indeed the galah was initially described as Cacatua roseicapilla. Early DNA studies allied the galah with the cockatiel or placed it close to some Cacatua species of completely different appearance. In consequence, the ancestors of the galah, the cockatiel and Major Mitchell's cockatoo were thought to have diverged from the main white cockatoo line at some stage prior to that group's main radiation; this was indeed correct except for the placement of the cockatiel. Ignorance of this fact, however, led to attempts to resolve the evolutionary history and prehistoric biogeography of the cockatoos, which ultimately proved fruitless because they were based on invalid assumptionssuch as what? to start with.

It fell to the study of Brown & Toft (1999) to compare the previously available data with their mitochondrial 12S rRNA sequence to resolve the issue. Today, the galah is seen, along with Major Mitchell's cockatoo, as an early divergence from the white cockatoo lineage, which has not completely lost its ability to produce an overall pink (Major Mitchell's) or pink and grey (galah) body plumage, while already being light in colour and non-sexually dimorphic. The significance of these two (and other) characteristics shared by the Cacatuinae had previously been explained away in earlier studies by strict application of parsimony on misinterpreted data.
Three subspecies are usually recognised. Slight variation exists in the colours of the plumage and in the extent of the carunculation of the eye rings among the three subspecies. The south-eastern form, E. r. albiceps, is clearly distinct from the paler-bodied Western Australian nominate subspecies, E. r. roseicapilla, although the extent and nature of the central hybrid zone remains undefined. Most pet birds outside Australia are the south-eastern form. The third form, E. r. kuhli, found right across the northern part of the continent, tends to be a little smaller and is distinguished by differences in the shape and colour of the crest, although its status as a valid subspecies is uncertain.