1. First, when you have a typical hair follicle, you'll find a small oil gland off of it. The normal cell cycle is for skin cells to slough off (aka- just basically fall off), and for new cells to be created underneath to replace them.
2. For some reason, within the hair follicle some of these cells will start to accumulate and clump together, rather than just going out as they usually do. As this happens, the oil production will increase some, the cells will clump together more, and the sides of the hair follicle will start to have inflammation. The inflammation contributes to more skin cells sloughing, clumping, more oil, etc. It basically becomes a downward spiral. The hair follicle starts to get clogged with oil and clumped skin cells.
3. As the hair follicle gets clogged with debris, you'll notice that rather than just having the walls of the hair follicle with inflammation, some of those cells will start to move into the debris as well. These are various types of white blood cells, and when they accumulate together you probably know them better as pus. Around this time you'll also have bacteria entering the mix. Typically this is p. acnes, which is usually just found around on your skin and such. It loves to eat oil, so this environment is perfect for it to thrive.
4. As the process spirals more and more out of control that hair shaft just fills up with everything. It will get big and swollen as it fills with sloughed skin cells, inflammation, bacteria and more sebum. You may even have the shaft rupture, allowing everything to infiltrate into the surrounding skin. That's not so bueno. If it is severe enough you might even see scarring later on down the road.
Retinoids will actually treat both of those issues. First, they help to normalize the rate at which the skin cells slough off. The cells that do slough off (at their newly improved rate) will have reduced charges and overall the levels of transglutaminase (and therefore cross bridges) are reduced as well. Blemishes under retinoid treatment are less cohesive and looser. You're less likely to have the whole follicle plug up and create the blemish, and then when it does the decrease in compactness means that other treatments will be able to penetrate into the blemish more easily. It's a win-win all around.
There are 2 main ways to treat the bacteria. The first is with Benzoyl Peroxide. Benzoyl Peroxide will create free radicals (read more about that in my antioxidants in skin care post), and those will directly attack and kill the bacteria. Keep in mind that due to the free radicals you'll see damage also to the area's skin cells, so there is a potential for speeding up aging.
The second way to treat the bacteria is obviously with antibiotics! Usually Clindamycin or Erythromycin are used. Both work equally well, though there's more risk of bacteria becoming resistant with Erythromycin. When used in combination with Benzoyl Peroxide the risk of resistance is lower. Both of these anti-biotics also work as anti-inflammatories. Both of these antibiotics need to be prescribed.
Finally, you'll sometimes see Sulfur (or Sodium sulfacetamide) used as the anti-biotic in over the counter meds. Sulfur can also work on the rate of skin cell sloughing, though it won't affect the charges and enzyme levels like a retinoid.
Alpha and Beta Hydroxy Acids are also great for helping to clear out the hair follicle. BHAs like Salicylic Acid are more effective because they are lipid soluble, so can penetrate into the sebum clogging the follicle much more easily. Note that Salicylic Acid is also anti-inflammatory.
There are a few ways to bring down inflammation to help acne. The first way is obviously steroids, but this should be avoided at all costs. You can have thinning of the skin from steroid use (steroid atrophy), but use of topical steroids can even cause "steroid acne". So, definitely no steroids! Instead, take advantage of the anti-inflammatory side effects of some of the other treatments, such as topical antibiotics and Salicylic Acid.
If you want to see immediate results, you should look for Salicylic Acid, Benzoyl Peroxide and steroids (with your physician). I've also had results using heat, with either a warm compress or my Tanda Zap.
So, that's acne and acne treatment made super simple. Obviously there's a lot more to this than I've explained (I'm but a simple pediatric intensivist, not a dermatologist), and when in doubt I recommend seeing your doctor. Most primary care doctors do treat a lot of derm issues (we did a lot of rashes and acne back in my general peds days), and if needed they'll refer you on to a dermatologist.