The point of a warm-up is to get blood flowing through your body, to get your heart rate pumping, and to get your body stretched out and ready to work up a stretch. Complete a warm-up before EVERY workout!
Cool downs serve as the exact opposite purpose: they're to lower your heart rate after it was raised by your workout. Your heart rate should return to normal (or close to it) during your cool down.
You can use stretching and/or yoga as part of your warm-up, or as your cool down.
10-Minute Warm-Up #1 – Full Body, with or without weights
10-Minute Warm-Up #2 – Mobility and Core
Quick 3-Minute Warm-Up
Stretch/Cool Down Routine #1 – Full Body
10-Minute Beginner Yoga/Stretching