Automatic and manual wind watches are both mechanical, meaning neither uses a battery, but they differ in how the mainspring gets its power. An automatic winds itself from the motion of your wrist through a rotor, while a manual watch only winds when you turn the crown by hand. Everything else, from the price to the case thickness to how the watch feels day to day, tends to follow from that one core difference.
How each movement actually gets its power
Inside an automatic watch is a rotor, a small weighted half-circle that swings freely as your arm moves. That swinging motion winds the mainspring through a series of gears, which is why an automatic watch worn daily rarely needs manual winding at all. Take it off for a few days and it winds down, but strap it back on and normal movement gets it going again without you doing anything extra.
A manual watch has no rotor. The only way the mainspring gets tension is you turning the crown, typically once a day, sometimes more depending on the movement's power reserve. There's no automatic backup, so if you forget to wind it, it stops, and you have to reset the time before wearing it again. Some people see this as a downside, others see it as a small daily ritual worth keeping, similar to the appeal of a manual coffee grinder over a machine that does it for you.
Both designs have been around for well over a century, and neither is a newer or more "advanced" version of the other. Automatics became widely popular in the mid-1900s as a convenience feature, but manual movements have never really gone away, especially in dress watches and smaller-diameter pieces where there's less room to spare.
Case thickness and feel on the wrist
Automatic movements need room for the rotor to swing, which generally makes automatic watches thicker than manual ones with a comparable movement. If you prefer a slim watch that sits low under a shirt cuff, a manual wind movement often gets you there more easily than an automatic will. This is one of the clearest practical differences between the two, and it's the reason a lot of classic dress watches are manual wind rather than automatic.
Manual watches also tend to feel a bit more direct, since every bit of power in the mainspring came from your own hand. Some people genuinely enjoy that connection, winding the watch each morning as part of getting ready, in the same way others enjoy the convenience of never thinking about it with an automatic. Neither feeling is wrong, it just depends on what you want from the ritual of putting a watch on.
Which one actually fits how you live
If you wear one watch daily and don't want to think about it, an automatic is the easier choice. It winds itself through normal wear, and as long as you're active enough during the day, it'll keep running without any extra attention. This is why most everyday watches, including something like a Seiko 5, are built as automatics rather than manual wind pieces.
If you rotate through several watches, or you like a slimmer dress watch for occasional wear, manual wind makes more sense. You won't be bothered by needing a winder to keep it running between wears, since you simply wind it by hand the next time you put it on. Neither option is more "serious" or better made than the other.
They're just two different answers to the same mechanical problem, and the right one depends on your habits more than anything else. If you're not sure, think about how many watches you actually own and rotate through, and let that answer the question for you.