There are two types of percussion instruments in music, pitched percussion & unpitched percussion. I like to refer to them as simply “percussion” and “melodic percussion”.
For example, a Timpani drum is tuned to a specific pitch (note). Whereas a bass drum is simply a low booming sound without a distinct note.
However, there are also instruments that are played as percussion, but that are still melodic. Meaning you can play melodies on them, or even harmonies and chords sometimes. For example a Marimba is a melodic percussion instrument.
Basically, a percussion instrument is really any type of instrument that makes a sound by you striking it, which could be with your hands, mallets or sticks, or any mechanism that strikes a part of the instrument that creates the sound. This is why the piano is actually considered a percussion instrument.
Let’s explore the main melodic percussion instrument used in modern orchestral music:
Piano
It may seem strange at first, but the piano is indeed counted as a percussion instruments. You press keys, which forces “hammers” to strike down on the strings, which creates the sound. However, even though it is technically a percussion instrument, it is mainly used for chords, harmonies, backing, arpeggios etc.
Celesta
This is another keyboard instrument, like the piano, but instead of strings there are metal plates that resonate when hit. You can compare this to a glockenspiel, but with keys, and more range. It has a very angelic and magical, shimmering sound.
Vibraphone
The vibraphone is a mallet instrument, meaning that you play it with mallets. The sound is created by striking metal bars, which creates pure bell like tones.
Marimba
The marimba is another one of the main mallet instruments, but it uses wooden bars instead of metal. This creates a softer and more mellow tone, but also more muted (short decay). This makes it more percussive and focused.
Xylophone
The xylophone used in the orchestra has wooden bars, like the marimba, but higher in range. The tone color and character is also different because the wooden bars are designed differently. The sound of the marimba is very soft and muted, while the xylophone is crisper, sharper and more defined.
Glockenspiel
The glockenspiel in the orchestra is sometimes called “orchestral bells”, since it has a very bell like sound. It is a mallet instrument, high in range, with metal bars that you strike with hard mallets.
Crotales
These are sometimes called antique cymbals, and they are indeed a set of tuned “cymbal like” discs made of brass or bronze. The discs are mounted on a stand, with a layout that resembles a keyboard, so that you can play chromatically.