The answers by LawrenceC and Teacher KSHuang are both good and seem to highlight important points, but for me, don't quite nail it. It might be just semantics, but I'll take a stab at it. My answer will overlap the others. BTW, this is not meant to denigrate lly's good answer, it's just that answer has a different focus.
Gather refers just to bringing close together things that are spread out.
- It does not imply any form of organization or order.
- It refers to many items aggregated in a non-selective way; the focus is dealing with them in the aggregate rather than individually, and just moving them all to one place.
- It does not address possession or retention of the items, or what you do with them afterwards.
You can gather people in a room (herd them to a common place), gather berries into a pile or bucket (and potentially just leave them there), gather information (refers to just pulling diverse sources to one place before processing or organizing it), gather fabric (pull spread-out fabric closer together), etc.
Collect could potentially be associated with gathering, but if it is, it would refer to what you do after gathering. The characteristics of "collect" are:
- You retain the items.
- It involves selection criteria, you are dealing with specific examples. It can be a single item or multiple items, but "collecting" is focused on selected items rather than a "mass quantity".
- There is often some form of organization or structure applied to the collected items.
If you collect arriving people, it is those specific people. You don't just herd them together and leave them there, you retain them (bring them with you).
Take your example of getting people to a meeting. Say everybody but John is in your vicinity. You might say, "lets all gather in the conference room for the meeting. Bill, will you please go and collect John?"
If you collect information, it is specific, targeted information, and you save it (retain it), in an organized or structured way.
If you collect fabric (as opposed to gathering it), there are specific criteria for examples you retain, and you organize them in some way.
LawrenceC's rainwater example is explained well. Gathering it just aggregates it; control and direction rather than retention because it won't stay aggregated on its own. Collecting it retains a specific sample (the water you manage to get into in the container).
For your junk example, a lot would depend on the situation. If you have an actual junk collection (i.e., save specific kinds of items), you might go somewhere to find items to add. In that case, you might say that you "went to collect junk", although people might be more likely to say that they "went to find junk to add to their collection".
Otherwise, if you went somewhere to simply make junk at that location neat by pushing it into one pile, you could say you "went to gather junk". If you were going for the purpose of taking specific junk with you, you could say you "went to collect junk". You might have to weigh which aspect most characterizes the activity. Is the "possession" aspect or the specificity of the items more relevant to the case?
This is an example where either term might apply. "Gather" isn't limited to one source location, and doesn't really address where the aggregate ends up. If your purpose is to aggregate the junk, load it on your truck, and drop it at a dump site, "gather" would apply because you're dealing with the junk en masse and any retention is incidental.
If you gather berries at two locations and then bring them home, you've still "gathered" berries, not "collected" them. The basis for distinction in the berries case is that the essential activity was aggregating berries on a mass basis. If the purpose was to look for very specific berries, "collect" would be more applicable. In some other case, different criteria might be the determining ones.
Not every usage case will perfectly fit one definition to the exclusion of the other. Sometimes it is a matter of which word is a closer fit. For example, gathering people in a room may involve a specific set of people, not just a random collection of people. Collecting water doesn't involve organizing it, and you need to take the concept of "specific water" loosely. Some cases come down to which term best captures the essential properties of the case.