SARMs, like others have said, are supposed to bind to the androgen receptor and give you steroid-like gains. They're supposed to have much more favorable anabolic to androgenic ratios meaning muscle growth without hair shedding, aggression, acne, shutdown....
Many sarms were designed to prevent muscle wasting in aids/cancer patients and others were designed to be male contraceptives. One thing I noticed was a total lack of appetite. Also a fair bit of lethargy. I had bloodwork done and both test and e were suppressed but liver values were fine, no elevation.
I have not seen anyone put on any 'steroid-like' size from sarms. But you can get very lean and hard without losing size.
I personally don't like them.
Mk677, like Van said, isn't a sarm but it's ok. It will definitely increase appetite and help you sleep but again it comes with brutal lethargy. Another non-sarm marketed as a sarm is sr9009, it's supposed to increase metabolism by manipulating your circadian rythm. You'd have to be an idiot to screw with your circadian rythm. Unless you like being an insomniac.
There was a video posted on here about some fat fuck who was totally shut down and was blaming lions mane and finasteride. I looked at all his YouTube videos and the guy ran S-23 a half dozen times in the past year or two. S23 is the strongest sarm and was designed as male birth control. The whole point of it is to shut you down without losing muscle. Nows this fat fuck isn't taking S-23 anymore and he's a flabby failure who sits around making YouTube videos blubbering about having a limp dick.