like a home stereo, i'll assume you have. Freq=Tone, Volume=Loudness/amplitude.
or a guitar ,
TONE.. like Freq of each string..A 440hz..G, B,E etc..
VOLUME being how loud you turn it up or slam the chords....
a compressor can help "smooth" out the soft and hard strums. kind of maintain electronically a steady volume of the guitar strumming.
a limiter is good for "protecting" your other equipment by giving it a volume "limit". So your buddy can turn up the fuzz pedal, the guitar to 11..but your limiter will decide/limit the volume into the recorder unit, for example...some call them Insurance from destroying equipment.
(usually set below 0db or below the Red line to prevent audio distortion).
a normalizer, it just kind of raises the volume on everything, like turning up the rec-level on a recorder unit... its like a Rec-volume knob imo.
Software you highlight the wave, tell it what volume Louder or Softer, in dB's...you can raise the volume on a whole song...and even a whole set of other songs!! all "Normalized" to the same level before burning a homegrown CDR MASTER Album of 10+ tracks. its slick, yes....in concept.
but it doesn't change anything, doesn't smooth anything...
and unfortunately, it doesn't magically make those 4qty crappy HR mixes I did in 1988 sound like a professional-mastered compilation all of a sudden...i'm still waiting for that software.