Adobe Illustrator - What is the difference between a Layer and a Group? [closed]

Question 1

Technically, there is very little difference. Internally, in the Illustrator artwork tree, both layers and groups perform the same function - they create a parent object that can contain one or more child objects. The differences lie in how they are used in a practical sense.

Groups are typically used to bundle artwork that you want to stay together when moving or scaling. For example, let's say you draw a door. It might contain inset panels and a door knob. When you want to move it, you can direct select each object or marquee select all the objects before moving, but that is slow and can also pick up additional art objects that may be in the way. Alternatively, if you group the door outline, the inset panels and the door knob, now all you need to do is click once on any of those objects to select the entire door.

Layers are typically used to manage visibility and stacking order. For example, let's say you draw a floor plan for a home. On one layer you might draw the walls. Then, on other layers you might draw electrical plans, furniture layouts, and plumbing plans. When you contract with an electrician, he may not need to see the furniture layout, so you would set the visibility of the furniture layer to false so that the printout would not contain any furniture. Also, you need to ensure that furniture is always drawn over walls, so you would use the layer panel to move all furniture at once above the walls layer.

Question 2

Incidentally, an easy way to select everything in a layer (and thus make them act even more like a group and muddy the waters of how these two things differ even more!) is to click on the small circle icon on the right of the layers panel. This selects everything, including groups, within that layer. Additionally, holding down alt and then clicking selects everything in all the layers.

A highly useful aspect of using layers is the ability to lock and hide elements while you're working. These are found on the left hand side of the layers panel. It should be clarified that you need to keep your drawing in good order as you go along to maximise the usefulness of this feature.

It is also worth mentioning that groups cannot be made up of elements from different layers. Attempting to do so will move everything to the currently selected layer and grouping it there.

The main use for groups I have found is for ease of selection if I have certain elements within a layer that will use the same appearance settings as each other, but that are different to the rest of the layer. Doing so allows me to simply click once to select the whole group and then apply the desired appearance settings.

Ultimately, how you use layers and groups is entirely a case of personal preference and what makes sense to you as to how to use the strengths of each e.g. as mentioned in the previous answer, grouping allows the same scale factor be applied to each element within, and also prevents them from losing their positions relative to each other if you use the arrange / align buttons on the top menu bar. Personally, I tend to use layers to break down what each type of element is representing; for instance I had to make a fair few maps studying architecture, so I would have a layer named contours, buildings etc. which would broadly use the same appearance so I could alter these quickly if I needed to by using the select method I mentioned above. Occasionally there would be groups within these layers, which admittedly I could have placed in additional layers instead.

Really, if it makes sense to you then run with it.