The single most useful thing to know about espresso is that it isn’t a kind of coffee. It’s a way of making coffee.
There is no espresso plant and no espresso bean. Bags labeled “espresso roast” are a roaster’s suggestion, not a different species — usually beans roasted a little darker because that tends to behave well under pressure. You can make espresso from any coffee you like, and plenty of very good cafés do exactly that.
What the machine is actually doing
Coffee is ground very fine and packed into a small metal basket. Hot water is then forced through it under real pressure — around nine times atmospheric — for something like twenty-five to thirty seconds. About an ounce comes out the other side.
Fast, hot and hard. That’s the whole idea. Every other brewing method lets water take its time; espresso rushes it, and gets a small amount of very concentrated coffee for the trouble.
The tan-colored foam on top is crema. It’s carbon dioxide left over from roasting, whipped into the coffee by the pressure. People read a great deal into it. It mostly tells you the beans are fresh.
Is it stronger than regular coffee?
Per mouthful, enormously. In total, no — and this is the thing almost everybody has backwards. A shot of espresso has roughly 63 mg of caffeine in about one ounce. A mug of drip coffee has roughly 95 mg in eight ounces. The shot is far more concentrated; the mug simply contains more coffee.
So the person ordering a double espresso to “wake up properly” is getting about as much caffeine as the person with a regular filter coffee, and finishing eleven minutes sooner. The numbers are in how much caffeine is in a cup of coffee.
Do you need a machine at home?
Almost certainly not, and we’d gently steer you away from it as a first purchase. Espresso is the least forgiving thing you can do with coffee: it needs a grinder good enough to adjust in very small steps, and it will punish you for a week before it starts being nice to you.
If you want good coffee at home tomorrow morning, a French press and decent beans will get you further than a cheap espresso machine will, for less money and considerably less swearing. If you want the espresso drinks specifically, the cheapest reliable route is a café.
And if you do go down the machine route eventually: buy the grinder first. We say this at some length elsewhere.