The time signature is the two numbers, one above the other, written near the beginning of the first staff line, specifying the number of s in a and the type of that gets one beat. It is not a fraction but is usually typed as one: 2/4, 3/4, 4/4, 4/8, etc. Read just the numbers: 2 4, 3 4, 4 4, 4 8, with no “by”, “over” or other words between them.
2/4 tells us there will be two beats per bar and the quarter note gets one beat; 3/4, three beats per bar with the quarter getting one beat; 4/4, four beats per bar with the quarter note getting one beat; 4/8, four beats per bar with the eighth note getting one beat, etc.
Tango is almost always in 2/4, sometimes in 4/8, rarely in 4/4 (but standard with Piazzolla and other post-1955 or so tango composers). Milonga is in 2/4 and Vals is in 3/4.
Beat is the underlying and regularly spaced pulse of the music, measured in beats per minute. There are a fixed number of beats in a bar, indicated by the time signature. Tango (2/4, 4/8, 4/4) has 2 or 4 beats per bar, vals (3/4) has 3 and milonga (2/4) has 2.
(There may be a sense of 4 beats even though the time signature is 2/4. Tango very often subdivides the 2/4 beat, doubling the count from 2 to 4, effectively using a 4/8 time signature. Some tango music is explicitly written in 4/8, most are in 2/4. See Tango Time Signatures and the Beat).
A bar or measure is a small segment of music containing all the number of beats as specified by the time signature.
A note is a sound or tone having two aspects:
1) The primary, auditory one, is pitch. Each note has a unique pitch, with a sound wave frequency measurable in hertz. In many parts of the world instruments are tuned to A at 440Hz.
2) The secondary, temporal one, is duration, called "time value" or "note value". When written or played each note has a specific duration, how long it lasts relative to the beat.
When pitch and duration are combined we get melodic shape and rhythm. Pitch creates melody and gives it direction, the melodic shape; duration provides the melodic rhythm.