There are many different types of file formats for 3D objects: AMF, VRML, OBJ, X3D, PLY, STL, and more.
There are also many different types of open-source and proprietary program-specific files: for Maya, Blender, ZBrush, SketchUp, Cinema 4D, Houdini, etc. These are programs in which you can import/export 3D files and programs that to varying degrees are able to design (manipulate, animate, explode) 3D objects.
Due to this complexity of program-specific files and various file formats, the one file format you should be concerned with is .STL - this is the "de facto" standard for getting virtual 3D objects to physical 3D printers.
(The list of Maya-compatible data import formats alone is mind-numbing! And you can see why the world of Computer Science is such a budding field: getting all these things to 'talk to each other' requires real expertise.)
But the .STL alone is not the 'end all' to the process. You do not simply 'plug in' the .STL into the 3D-printer and it 'just works' - the .STL file is simply the shape of the object, its outline, its contours. Whether your file is a simple cylinder or a complex imaginary space vehicle, the .STL alone is just the shape - and that shape is hollow on the inside.
Once you have the .STL you need to prepare it for 3D-printing. You have to bring the .STL into a software environment and set it up for 3D-printing. We call this process import. (There is a reason we don't call this 'opening' the file, which we'll get into later.) And then you have to slice the file to turn it into code 3D-printers can understand.
To recap:
In modern operating systems .STL files are now natively viewable - that is you can 'view' the contours of the model without sophisticated software. You are only 'viewing' the file, like the 'preview' of a JPEG image or 'quicklook' of a MP3 or MOV file. It will look something like this when you 'double-click' the .STL file:
You can use your mouse or trackpad to pan around the image.
PRO TIP: If you spend any significant amount of time with 3D-modeling, you will want to invest in an external mouse! You will find using 3D software and doing work with 3D models a LOT easier with the aid of an external tracking device versus the limitations of a trackpad.
Continuing with the treefrog model, when the model is static we can zoom in and see its contours:
You can see the round eye isn't truly round, but is rather a geometric mesh of polygons. (The slight indentation is a representation of the frog's pupil, not to be confused with the curved contour of the cornea.) Comprised of faces, edges, and vertices, the shapes eventually form to create the model. But not a model with smooth-edge resolution. When the 3D model is exported to .STL you will decide at that time how many vertices to use in the .STL - the number of vertices will determine the number of polygons, the various shapes we see here. More vertices also take more memory and creates a larger file size, so therefore more vertices is not necessarily "better."
More importantly: If your eventual purpose of the model is a 200 micron resolution print, perhaps a lower number of vertices is acceptable.
(Spoiler: it is.)
Jumping a bit ahead, when the .STL is eventually prepared for printing it loses some of its resolution anyway, due to the nature of turning something comprised of polygons into something in the real world, a physical object. The 3D-printer won't 'move' or 'print' each of the polygons - instead the 3D-printer software further interprets the contours of the .STL as a series of moves while extruding plastic layer by layer. Your model will thus translate to something like this at the preview stage:
In this preview, the orange tubular paths are the paths of the print head and the user-specified layer height of the extruded material. (Don't worry about the yellow & red at this point - in fact don't worry about these colors at all, because they are different for each piece of software.) This preview is up close - so while you may perceive a 'gap' between the orange paths remember each of these layers is as thin as a human hair. As you can see in the actual print (below), the layers stack nicely to form a cohesive print.
But another point on the 'preview' function: this is an important step during the setup process, because it allows you to look for potential defects in your print before you go to print. You can view the path of the print head and all of its anticipated moves, layer by layer. If you see anything wrong now, you can go back to your original model and fix the error - in the .STL or the print settings - before sending the file to print. This could potentially save you hours or even days of work once you know what to look for. Conversely, if you get a failed print, you can go back to your file and the printer settings to preview the print and compare the failed print with that layer in the preview to see if you can deduce why it failed.
For this reason it is important to keep track of your .STL files along with the printer's "project" files! It cannot be stressed enough how important this step of the printing process is: it is very important!
Markets and repositories for .STLs are now common. Repositories ("repos") can be thought of where people just put stuff, like storage; and markets are as the name implies places where things may potentially be for sale. There are some absolutely stellar free sites (repos) out there, but you may find yourself fond of a specific designer or maker, and that person may only make their files available on a premium or pay-per-file market. There are a few good 'top-level' websites (repos & markets), and then like most things you will always find users in other channels just giving things away on Reddit or sharing with their tight-knit community on Discord. Wherever you get your files from - or want to share your own! - be sure to give credit where it is due. With that said, we like:
Neither a market nor a repo, r/3Dprinting is Reddit's channel for 3D-printing everything - files, news, stories, etc. It has a mere 471k global members.
OK so .STLs are outlines of 3D shapes - the contours of the shape.
In order to turn a .STL into code that can be interpreted by a 3D-printer, we need to import the file into a slicer program & slice the file, layer by layer.
Import -> Slice are our next steps!
