How the Piano Changed Music
The concept of using dynamic markings (which indicate the relative loudness of different sections in the music) in sheet music came about around the same time as the piano — for good reason. Before the invention of the piano by Bartolomeo Cristofori in 1709, most composers were confined to writing most of their pieces for either the harpsichord or the clavichord; neither instrument had the capability to play both soft and loud sounds easily.
The reason for this is that the basic internal design of both the harpsichord and the clavichord follows the design of a stringed instrument. However, instead of having one's fingers in direct contact with the string, as with a guitar or a fiddle, harpsichords and clavichords are fitted with a plucking mechanism inside the instrument itself. When a certain key is pressed, the corresponding internal string is plucked by the mechanism. No matter how hard or soft you press the keys of either instrument, the resulting volume is pretty much the same. Therefore, clavichords were used for quiet pieces of music that required lots of vibrato (quavering quality to notes that usually comes from a stringed instrument), whereas harpsichords were used for louder, brighter-sounding pieces.
Although the piano looks a lot like these two other instruments, it's really very different. The piano incorporates a hammer-and-lever mechanism that strikes each string with the same force as the human finger on the piano key did; this is why the piano is considered a percussive instrument. The piano made both quiet and loud sounds possible on the same instrument, and, therefore, in the same musical piece. This is why the piano was originally named the gravicèmbalo col pian e forte, or "harpsichord with soft and loud." The name was later shortened to pianoforte and, finally, to piano.
Since its inception, the piano has been the universal tool of choice for composing music, because almost every note you would ever want to work with is present on the keyboard, right there in front of you. Most pianos have at least 7 octaves to work with, and concert pianos can have more than 12 octaves.
You want to compose music for an oboe? The lower registers of the piano work quite nicely. Pieces written for strings can be easily hammered out in the middle and upper registers. And unlike most other instruments, you can hit chords and multiple notes simultaneously on a piano, which works well for trying to figure out how that multi-instrument orchestral piece you're writing will eventually sound.
You can blame the fact that the piano was invented by an Italian for the bulk of tempo and dynamic markings being Italian words. From day one of the piano's release to the initially Italian market, composers were finding new ways to write music on this spectacularly flexible instrument.

Piano Glossary
accidental
A sharp or flat not in the key signature.

Piano Glossary
bar line
A line that divides music into measures, breaking up the musical paragraph into smaller, measurable groups of notes and rests.

Piano Glossary
bass clef
A musical symbol that indicates lower the notes, generally played with the left hand.

Piano Glossary
beat
The steady pulse of a piece of music; what you tap your foot to.

Piano Glossary
C position
Placing your right-hand thumb on middle C and your other right-hand fingers on the four successive white keys.

Piano Glossary
chord
Three or more notes played at the same time.

Piano Glossary
Circle of Fifths
An order that starts with no sharps and flats and cycles the ring of keys to all 12 keys.

Piano Glossary
clef
A musical symbol that tells you the names of the lines and spaces on the staff.

Piano Glossary
fingerings
Numbers written above the notes that tell you which finger to use for each note.

Piano Glossary
G position
Placing your right hand on the G above middle C and your other right-hand fingers on the four successive white keys.

Piano Glossary
grand piano
The largest piano. A concert grand is 9 feet long; a baby grand is about 5 feet.

Piano Glossary
grand staff
The treble clef and bass clef joined together with a brace at the start of the left side.

Piano Glossary
home note
The base note of a piece of music. All the notes in a song have a relation to the home note based on how close or far they are to home.

Piano Glossary
interval
The distance between any two musical notes.

Piano Glossary
key
A set of notes that corresponds to a certain scale.

Piano Glossary
key signature
Notation placed just after the clef on every line of music to tell the performer what key the song is in.

Piano Glossary
ledger line
An imaginary line running above or below the staff, extending the five-line staff to represent notes above and below the staff.

Piano Glossary
measure or bar
A batch of notes with a specific number of beats — most commonly four beats — that helps a performer keep time.

Piano Glossary
middle C
The 40th key of a piano, close to the center of the keyboard. The first key a new pianist learns the placement of.

Piano Glossary
MIDI Musical Instrument Digital Interface
A device that allows communication between electronic, digital equipment.

Piano Glossary
notehead
The printed representation of a note. A notehead is oval and may be solid or open, an attached to a stem or not.

Piano Glossary
octave line or ottava line
The line that tells you to play the indicated note or notes an octave higher or lower than written.

Piano Glossary
root note
The lowest note of a chord.

Piano Glossary
scale
A series of notes in a specific, consecutive order.

Piano Glossary
staff
The five lines and four spaces music is written on.

Piano Glossary
tempo
How fast or slow the beat is.

Piano Glossary
time signature
The indication of the meter of a piece of music.

Piano Glossary
tonic
The bottom note of a scale.

Piano Glossary
treble clef
A musical symbol that indicates higher the notes, generally played with the right hand.

Piano Glossary
triplet
Three notes per beat.

Piano Glossary
upright piano
A piano that sits upright against a wall. Also called verticals, they vary in height from the spinet up to full-size uprights.