A music note is a way of writing and considering a single unit of Western musical sound. Each note has a pitch and rhythmic indication that illustrate what the note sounds like and how long it lasts.
The term "pitch" refers to the frequency level of the sound. A foghorn, for example, has a relatively low frequency and pitch within a human's audible range, where a whistle has a high frequency and pitch.
Western musical instruments play pitches at assigned frequencies. When the frequency is slightly altered, usually by a poor singer, the pitch sounds wrong, flat or off-key to you.
You can also determine pitch by reading notes. Notes are presented on a five-line horizontal bar called a staff, which illustrates pitches within a given range. There are different staves for different instruments, to illustrate the varying ranges of these instruments. For instance, a male singer usually sings from a lower-level staff than a female singer.
The term "rhythm" refers to the length of time during which a note or series of notes is played. Most Western music is divided into beats, units of time separated apart to such an extent that you can usually clap once on each beat.
Most Western music is written so three or four beats makes a larger unit. When written, this larger unit is a bar. The start of a bar is audible by the downbeat, or the beat upon which emphasis is placed in the performance. In written form, the bar is a vertical line interrupting the five horizontal lines of the staff.
Individual notes are assigned rhythm based upon the amount of the bar their duration lasts. A whole note, for example, takes up an entire four-beat bar. A half note takes up two beats, and a quarter note takes up one beat. The bar can be divided as much as necessary, into very detailed minute temporal moments during which a higher number of notes can be played.