Front ensemble

In a marching band or a drum and bugle corps, the front ensemble or pit is the stationary percussion ensemble. This ensemble is typically placed in front of the football field, though some groups will work the front ensemble into a tight pod onto the marching field. Some high school marching bands opt not to march any percussion instruments, but instead have a "full" front ensemble. 
Originally, the front ensemble consisted of keyboard percussion and timpani, the marching versions of which are heavy and awkward. Groups began adding more and more traditional percussion instruments to the pit, and in its modern form, the ensemble may contain any type of percussion instrument from cymbals, gongs, and drum kits to Afro-Cuban percussion such as congas, bongos, claves, and cowbells, to African percussion such as djembes. 
The main emphasis of the pit in drum corps style groups are the mallet instruments: marimba, vibraphone, glockenspiel, chimes, crotales, and xylophone. Some marching band circuits also allow non-standard instruments (such as the violin) or electronic instruments (such as synthesizers and bass guitars) in the pit. 
In indoor drumline, the front ensemble may not necessarily be placed at the front as the name suggests. The show designers place the pit where it would be most effective for the show.
The term "pit" comes from musical theater, where the accompanying orchestra sits in the orchestra pit.
History.
Originally the only mallet instruments allowed to be marched were timpani in 1972, glockenspiels, and xylophones in 1974. In 1977, marimbas and vibraphones were allowed to be used, but they still had to be marched. Overtime, people began to realize the physical strain of carrying these large, awkward instruments, and they were allowed to be grounded in 1978. This allowed extended techniques and higher quality instruments (like pedal timpani and 4-octave marimbas) to be used. 1982 was the last year marching mallet instruments were used in competition. Now it was not uncommon to see an assortment of instruments previously only used in other genres such as drum set, congas, bongos, and various types of mounted cymbals. Due to the advent of stationary percussion, marching cymbal parts are now commonly written into the front ensemble parts. The use of electronic instruments in marching band is controversial and divisive within the marching band community, and was prohibited outright by Drum Corps International until 2008, when it was passed in an 11-4 vote.
Instrumentation.
Instrumentation for the front ensemble in not completely standardized, and not all bands have access to certain instruments. High schools may not have enough funding for even one of each mallet instrument; a typical DCI World-Class corps will have an instrumentation similar to: 
Pitched Percussion.
4-6 Marimbas 
4-6 Vibraphones 
1-2 Glockenspiels 
1-2 Xylophones 
0-2 Crotales 
0-1 Tubular Bells 
1 Set of Timpani (4, 5, or 6 drums) 
0-1 Steel Drums 
Non-Pitched Percussion.
0-1 Drum set
0-2 Rack Percussion (Percussion Stations)
0-2 Bass Drums/Tam-Tams
Various Cymbals/Crash Cymbals
Ethnic Percussion (Djembes, Congas, Bongos, Timbales, etc.)
Electronic Instruments.
0-2 Synthesizers/Samplers
0-1 Bass Guitar
0-1 Electric Guitar
Electric Drums