FWB
The defining idea is friendship combined with sexual intimacy. Romantic partnership is not assumed.
Casual dating
Casual dating involves going on dates or exploring romantic/sexual connections without an assumed exclusive long-term commitment. Friendship may develop, but it is not the defining feature.
Where they overlap
Both can be non-exclusive, both can involve sex, and both can become more serious if everyone involved wants that.
Why the distinction matters
Someone comfortable with casual dates may not want sex with a friend, while someone open to FWB may not want to “date” that person romantically. Use the label that best reflects the actual expectations.
Labels describe; agreements define
FWB, hookup, casual dating and situationship are useful shorthand, but none of them automatically answers whether a relationship is exclusive, how often people communicate, whether sex is involved, or whether either person wants commitment later. Two people can use the same label and still have materially different expectations. Treat the label as the beginning of the definition rather than the complete definition.
Choose the comparison that changes a decision
Relationship labels are most useful when the distinction affects what you do next. If the question is whether to date other people, discuss exclusivity. If the question is whether this is becoming romantic, discuss intention and expectations. If the question is sexual health, discuss partners, protection and testing. The practical issue should drive the conversation.
Labels help describe a relationship, but expectations are specific to the people in it. When something important changes, a direct conversation is usually more useful than trying to decode a rule from the internet.