The 20th century orchestra has been enormously varied in line-up, constitution and purpose. Many large ensembles, representing all kinds of musical styles and aesthetics, have adopted the name 'orchestra' as opposed to 'band' or 'ensemble'. The 'classical' orchestra, with its large body of strings, remains probably the defining configuration, but even this has been altered and expanded in ways unimaginable in the 19th century. Such orchestras perform film and television music, as well as concert music, and their repertoire encompasses a great range of styles and historical periods.
First, there were new instruments. One early major addition was the saxophone, which in fact appeared during the 19th Century, but took a while to become established as an orchestral instrument. The heckelphone was a bass oboe (rarely used today) and the contrabass clarinet was also constructed (again, this is still a relatively rare instrument). Keyboard instruments, such as celeste and piano, became regular members of the orchestra, and various electronic instruments (e.g. ondes martenot, synthesizers) have been added, as well as tape.
Probably the most dramatic development has been in the percussion section. It is not uncommon to see huge amounts of percussion used in modern orchestras, including a range of tuned instruments, unusual or exotic untuned instruments, and many effects. Thus, Igor Stravinsky's Rite of Spring (1913) included timpani, triangle, tambourine, guiro, 2 antique cymbals, cymbals, bass drum, tam-tam (as well as quadruple woodwind, including two bass clarinets, and a large brass section) and by 1926, Edgard Varèse included 39 tuned and untuned percussion instruments in his massive orchestra for Arcana.
However, there was also a reaction against the use of such large forces, which were seen as being associated with 'overblown' Romantic music.Stravinsky himself was one of the leaders of this reaction, often turning to non-standard orchestral line-ups to explore new musical possibilities. This may be gauged from his comments following a performance of his Concerto for Piano and Winds in 1924.
"I remember that I was reproached on the subject of the constitution of the orchestra, which was said to be "incomplete" because of the absence of strings (except for the double bass). The unfortunate critic did not know at the time that there is such a thing as un orchestre d'harmonie. It is this orchestre d'harmonie (the concert band) which I have chosen for my piano concerto, and not the symphonic orchestra, as an instrumental body more appropriate for the tone of the piano. Strings and piano, a sound scraped and a sound struck, do not sound well together. Where as piano and winds, a sound struck and blown, do sound good together."
This focus upon timbres and successful blend is very characteristic of 20th Century orchestration. The orchestra as a body steadily became a 'sounding object' or a large-scale timbral palette, and many composers explored different and individual facets of this in their work. All this means that it is very difficult to make any general statement about what may be regarded as standard orchestral practice, but the 19th century tendency to introduce new instruments into the orchestra continued unabated, to the point where pretty much any instrument is now acceptable.