Envelopes don't change the waveform (like saw to square or anything else). They change volume, or pitch or filter cutoff, or any other synth parameter overtime.
Envelopes are oneshots. If it's an ADSR envelope (the most common kind) then when you press the key on your midi keyboard, it does the attack part of the envelope for however long the attack time is set, then the decay part for as long as the decay time is, then stays at the sustain level for as long as you hold the key. When you let go it does the release phase.
By contrast an LFO just cycles through the LFO shape forever. As long as the key is being played, the LFO keeps going around and never reaches a sustain where no more changes get made.
TL;DR: ADSRs only go once through, LFOs keep going around and around as long as the sound is playing