I’m not a lawyer (and this is not legal advice), but I’ll take a stab at the question.
Motions for Dismissal and Summary Judgment have one obvious thing in common: disposing of all or part of a case, but they are actually different.
A “motion to dismiss” asks the court to decide that a claim, even if true as stated, is not one for which the law offers a legal remedy.
A “motion for summary judgment” asks the court to decide that the available evidence, even if taken in the light most favorable to the non-moving party, supports a ruling in favor of the moving party.
A motion to dismiss first assumes the facts claimed in the claim are true, but irrelevant. A federal judge in Ohio with two companies in Ohio Plaintiffs vs. an Iowa individual and corporation could decide that there is no standing to sue over claims made about hypothetical shopping habits of a non-joined party that lived and shopped in California.
A motion for summary judgment assumes the facts are interpreted as favorable as possible to the opposing party, but that even that means the mover is entitled to judgment as a matter of law. Let’s say that California party is joined and in a relevant jurisdiction, and the claim is about whether or not said party bought a house, and whether what defendant said about the alleged purchase constituted defamation. The judge could rule that saying someone bought a house when they leased it isn’t inherently defamatory, and the facts of the case, taken together with the law, don’t support a claim of defamation. Hence, summary judgment.
Courtney Milan’s Explanation
[Courtney Milan has a clearer non-technical explanation in this post.