There are nests and also individual spawn points, and every now and then they change, the nests all change their locations around every month.
For example, near an area where i hang out a lot there recently was a Doduo spawn, it was 2 or 3 spawn points, and every few hours a doduo would show up on the nearby list. Whenever it showed up we knew that it showed up at one of those nearby spawn points.
Basically each spawn point has a list of pokemon it randomly picks from, a short list of maybe 8, and then spawns one of these, but some pokemon are more likely than others.
Nests are generally large areas with many spawn points, and for a while, all of them have the potential to spawn a specific pokemon. So you have like a large park where you just keep running into tons of bulbasaurs, you walk around there and you know you will find a few bulbasaurs each hour. Then all nests reset after a month or so, and the same area is now crawling with Mankey.
Known nests always spawn a lot of one specific pokemon, and after a nest rotation it makes sense to check tem all out to see if you find something cool. If you find one that spawns something you need, you keep coming back until the nests rotate again or until you caught enough of it.
Some pokemon of which i still need more, for pokedex and to make strong gym pokemon, would be charmander and abra and machop and exeggcute. All those can show up in nests, like many others.
As Visualjae pointed out, some rare pokemon simply have no fixed spawn points or nests. You need luck. Some of those are Snorlax, Lapras, Chansey, but also a few others like Porygon, Tangela, Aerodactyl. Those just insist on being super rare and refuse to show up in any pattern.
Finding nests or hearing from others that a particularly awesome nest is active somewhere can be a really big thing, especially if it features a really strong gym pokemon.
Some strong gym pokemon where real nests exist and you can evolve them by catching many of the lower-stage evolution in a nest would be Jigglytuff, Polywrath, Slowbro, Exeggutor, all the eevee-stuff, Arcanine.