For me, a squish is an aesthetic+personality attraction that I do NOT want reciprocated. I would describe a crush is an aesthetic+romantic attraction that I would hope is reciprocated because I would want it to develop into a romantic relationship.
Squish: I want to look at you, sit with you, stay up 'til 2am exchanging bad puns with you.
Crush: I want to hold your hand.
It's really, really easy for me to experience squish feelings, instantly freak out, and jump right to "omg, I have a crush on them, what do I do, am I not acearo anymore what happened who am I" and that is incredibly frustrating.
I have this thing that I do whenever I need to differentiate between a squish and a crush. I first picture the person in my mind as best I can and then picture kissing them and doing...other things. I usually don't get to the other things before being completely disgusted and overwhelmed, which reaffirms that I do not have a crush on them. I think that's the difference: do I want to have a physical, romantic relationship with them? If not, it's a squish and I just enjoy hanging out with them.
The people I've had squishes on are people I really enjoy hanging out with. I kind of look up to them too, for the most part. They've all been extremely smart people that know a lot about subjects I'm not particularly well-versed in, so we get to swap knowledge.
The way I see it, a squish is when you want to spend time with a person and get to know them better. A crush is when you want the person themselves.
With romance, there's usually an expectation of exclusivity. A sense of "I belong to you, and you belong to me." This is why cheating on your spouse is deeply emotionally painful in a way that having multiple close friends is not. It's a different kind of love.
Even having a crush on someone you're not dating will usually come with feelings of jealousy if they're interested in someone else. This doesn't happen with squishes.
Granted, I'm aro-ace, so I don't really get crushes. But this is how I understand it.
For me personally, when I have crushes my heart does this little squeeze when I see them or hear their name (if I’m spending a prolonged period of time with them then this usually only applies to when I first see them but it can also happen throughout depending). I’m still pretty anxious/emotionally underdeveloped so whenever this happens it means that I avoid eye contact and go to lengths to be nonchalant. I also think about them a lot and have a little trouble stopping, and I might randomly smile at the thought of them. I also get nervous before I say their name because a part of me is scared my voice will come out giddy if I do.
Squishes don’t really have those elements. There are some elements that are common to both squishes and crushes (planning my day/week to be around them more, wanting to text them a lot, doing whatever I can to make them smile, etc.), but I can’t think of any traits that are unique to just squishes and not crushes, except that squishes are easier because I’m a little less nervous.
Also, jealousy is different. With squishes, I can be jealous when they spend time with other people, but it’s more of a wistful (man, I should have been there) sort of thing, whereas with crushes there’s a little more pain there. Like, I guess the difference is that I wouldn’t mind if someone I’m squishing on (is that a phrase) spends time with other people if I could also be included. But with crushes, being included isn’t enough—there’s a spiteful part of me I try to ignore that wants everyone else to go away forever lmao.