Chemical exfoliants come in two broad families, and the difference between them is mostly about where they work. AHAs (alpha hydroxy acids) are water-soluble and act on the skin surface. BHA (beta hydroxy acid, almost always salicylic acid) is oil-soluble, so it can travel into an oily pore and work from inside it.
What AHAs do
The common AHAs are glycolic acid (small, fast, more active) and lactic acid (larger, gentler, more hydrating). They loosen the bonds between dead surface cells, which over time improves tone, smoothness, and the look of dullness and uneven texture. They suit skin that reads dry, rough, or uneven more than skin that reads congested.
What BHA does
Salicylic acid is oil-soluble, so it gets into the pore lining and clears the mix of oil and dead cells that leads to blackheads and clogged, bumpy texture. It is also calming at low concentrations. It suits oilier, congestion-prone, and blackhead-prone skin.
AHAs work on the surface; BHA works inside the pore. Choose by the problem you actually have, not by which acid is trending.
Which suits which skin
If your main concern is tone, texture, or dryness, reach for an AHA. If it is oil, blackheads, or congestion, reach for BHA. Skin that is both can use one of each on different days. Sensitive skin usually does better starting with low-strength lactic acid or a low-percentage salicylic acid rather than high-strength glycolic.
Can you use both?
Yes, but not necessarily in the same step or on the same night when you are starting out. Over-exfoliation is the common mistake: more acid is not more result, and a stripped barrier undoes the benefit. Build one in, read the response, then consider the other.