Rhythm: Rhythm is the way music moves through time, its movement in time. The sounds of music may move through time by combining short and long sounds or by stringing together a series of short or of long sounds or by any combination of these. The various combinations of ways for music to move through time are rhythm. In "Stay," the rhythm combines whole notes with doubles and triplets creating a seemingly steady rhythm that is actually broken into unsteady smaller bits.
Tempo: Tempo is the pace of the music; it's the pace of how quickly or slowly the beat moves along. Tempo is external to rhythm in that tempo may speed up or slow down without affecting a change in the rhythm, although the rhythm will be hastened or drawn out. In "Stay," the tempo is "moderato," or a tempo that is neither quick nor slow. Moderato, or "moderate," is said to be natural to a walking pace or a heartbeat pace.
Beat: Beat is a single unit of the division of musical time of a piece of music. The beat is also thought of collectively as the collection of the repeating beat. A single beat the dominant note duration the music is written in, while the collective beat denotes the repetition of the beat pattern. For example, a quarter note may represent the dominant beat duration, while a pattern of two beats, for 2/4 time, may represent the repetition of the beat pattern: a quarter note in one beat and a collective repetition pattern of two quarter notes: A single beat is a quarter note (or its equivalent), and the beat pattern is the quarter note (or its equivalents) repeated twice. An alternate beat using a quarter note would be 4/4. "Stay" is written in 4/4 with one quarter note per individual beat (or its equivalent, e.g., two eighth notes) in a collective pattern of four repetitions.
Pitch: Pitch refers to the vibrational frequency of each individual note. Notes are set to an international frequency standard, with 'a' at 140 Hz, since 1939. When a conductor says to a singer or singers, "Mind your pitch," it means that the voice(s) is sliding below or above the true (standardized) pitch. Some professional pop and country music singers do sing off pitch, for example, country singer Roy Orbison tends to sing below the pitch and Larry Graham of "One in a Million You" fame has a tendency to pitch low now and then. In "Stay," Sugarland's Jennifer Nettles is correctly pitched with the instruments, though she seems to drop her pitch now and then on dramatic words, seemingly for dramatic effect.
Melody: Melody is the ordered movement from pitch to pitch. In "Stay," the melody is a simple though beautiful one in the range of normal speaking tones.
Harmony: Harmony is the compatible frequency resonance of two or more notes sung simultaneously. Disharmony, or dissonance, is the incompatible frequency resonance of two or more notes sung simultaneously. "Stay" is built upon gentle compatibility of the frequency resonance of the notes sung by the singer with the full range of melody and harmony notes played by the guitar.
I would say the dynamics of this song are mezzo-piano and mezzo-forte. There is a homophonic texture between the acoustic guitar and the singer's voice because it features two or more notes sounding at the same time in harmony. The song is in the key of E major, which I figured out by looking at the key of matching piano sheet music. Hope this helps!