A jointer is an electric planer designed to surface the edges of a board. It does it's job in a very similar way that a thickness planer surfaces the top and/or bottom of a board.
A common reason for surfacing the top and bottom is usually associated with achieving parallel surfaces with a desired thickness of board between them.
A common reason for surfacing the edges of a board are to insure that they are square with the top and bottom. I have heard it is called a jointer because this step is a critical in the preparation of 'component' boards before 'jointing' them up into a panel. Whether this is correct or not, surfacing edges has been called 'jointing' since at least the mid 1800s when it was used to describe hand planes with longer beds (over 20").
Jointing is among the many tasks that I feel table routers can do exceptionally well. It is very unlikely another summer will pass before I get my first thickness planer, yet I'm not entirely certain I will ever be willing to dedicate the floor space required to a jointer, on the premise a router can do this for me.
Dedicated jointer machines are common up to thicknesses of 6", nearly twice the size of longer straight bits for routers. Their bed performance is generally superior to the average router's fence. I intend to craft up a custom fence specifically tailored to accommodate jointing if/when required. Currently, my need to joint boards is low enough that my hand planes can get the job done fast enough. (working mostly with softwoods has some bearing on that).
I have also seen table saw blades marketed as being 'planer' blades. While I haven't tried any of them, I have had great results using both 60 and 100 tooth 10" blades to smooth and square edges of a board. I'm also told that taking a cut that is less than the width of the blade can lead to premature warping of the blade.
Whether or not you should get a dedicated jointer is a complex decision. From the description of what you want to do, you will need to do a lot of edge surfacing, so it could be the right answer, especially if floor space and budget permit.
wbh1963 is flowing with the grain in Arlington, Washington, USA
A day without curls is like a day without sunshine!