A Path is a series of plotted points (anchors points) with line segments between the points connecting each point to another point. A Path is made of minimally 2 anchor points and 1 line segment. However, there's no limit to the maximum number of points and segments a path may contain.
An Anchor Point on a path can only have an "in" line segment and an "out" line segment. In Illustrator, no anchor point can ever have more than 2 line segments connected to it. An anchor with no line segments connected to it is just an anchor point and not a path. A Path must contain at least 2 anchor points and 1 line segment.
I may go so far as to say every object in Illustrator which is not a raster object, has an anchor point. And most objects have at least 1 path. - Basically, if it's not just a single anchor point, it kind of has to be a path in Illustrator. (Raster objects have bounding areas, but not really any paths.)
Since most everything in Illustrator has a path, an easier method of thinking or defining objects is via Shapes and Paths.
- And Open Path is defined as a path with separate anchor points on each end. If an anchor point only has one line segment connected to it, that anchor is seen as an "end" point, and thus the path is "open". If a path contains one "end" anchor point, it must contain anther end another point. There's no such thing as an open path with only one end point. Traditional terminology is to call open paths merely Paths. No further terminology is really necessary. It's a series of anchor points connected by line segments, so it's just a "Path". "Open Path" is sometimes used to further clarify the construction of the path.
- A Closed Path is defined as a path where all anchor points are connected to each other. If all anchor points on the path have two line segments connected to them, then the path would be seen as "closed". Shape is a traditional term for a closed path. Or as you phrased it... "outline". A shape is not required to have a fill or a stroke. The important aspect to defining a "Shape" is that the path is closed.
(An easy metaphor may be to think of paths like a water balloon... Imagine the path's fill as water... if the path would leak water, it's open. If the path would contain water without leaking, it's closed).
- Stroke indicates a path with an appearance added to the path itself. The path is used as a spine to define where the stroke should be placed.
- Fill indicates a path with an appearance added to the area the path encompasses, not the path directly. The path is used as a container for the fill rather than a spine.
- You can combine these and have both a stroke and a fill using the same path, whether that path is either open or closed.
Okay.. with all that explained.....
The Pencil Tool draws either a path or a shape. By default the Pencil Tool draws a path -- a series of anchor points with line segments connecting them, leaving two end anchor points.
- You can however cause the Pencil Tool to draw Shapes. If you hold down the Option/Alt key when you lift up with the Pencil Tool, the two end anchor points will automatically be connected to one another, creating a shape rather than a path.
The Blob Brush creates a Shape in all instances. When you draw with the Blob Brush the preview you see is the spine. This looks similar to a path, however, it is merely the application's way of showing you where you've already moved the depressed cursor. The spine is a preview only and not an actual object. When you let go of the mouse, the spine disappears and a shape is created using the spine (and the Brush appearance options) as a guide.
The Blob Brush is really just a Calligraphic Brush with an "auto expand" feature. The Blob Brush draws no differently than any of the Calligraphic brushes in the
Brush Panel (using the Brush Tool).
- The only differences between the Blob Brush and a Calligraphic Brush (with the Brush Tool) are:
- With A Calligraphic Brush/Brush Tool, the "spine" preview you see while drawing will become a path when you lift up on the mouse (open path) and no "shape" will be created.
- With the Blob Brush, as soon as you let go of the mouse, the artwork is immediately expanded to a shape (closed path) and there will be no path created from the spine preview.
You can see the Blob Brush calligraphic options by double-clicking the Blob Brush Tool in the
Tool Panel. They are the same options you see if you were to adjust any of the Calligraphic Brushes in the Brush Panel. You can just as easily draw using the Brush Tool and a Calligraphic Brush, then when you're done choose
Object > Expand Appearance from the menu, and you get the same results as you would if you were to draw using the Blob Brush.
As for when to use which....
That's all very subjective and dependent upon the artwork, your working style, and final output needs. There are few definite restrictions for when to use a Shape or when to use a Path.
In fact the only real construction aspect to be aware of is:
In Illustrator, it is advisable to always close any path with a fill. Open Paths with fills may appear okay as you work. However, open paths with fills can be problematic if the artwork needs to be output or be used in other applications. So, if the path has a fill, it should be a Closed Path and thus a "Shape", not just a "Path". (again the water balloon, if the fill is seen as water... it shouldn't be able to leak.)
Strokes can be on open or closed paths without an issue. The requirement of closing the path is not true for strokes. Open or closed paths with strokes are never a problem.
Beyond this... you are pretty much free to use whatever works for you. Either the Pencil Tool or the Blob Brush are fine to use, whether or not the resulting artwork is what you want is your call. There's no technical reason to avoid using one or the other. In some cases a path is easier to edit than a shape, but that's up to you to determine.
Sorry this got so long... I may have gotten too verbose and explained too much (which you probably won't even read :) ). Hopefully it clears something up for you. :)