It's used to refer to:
* Small pieces of bread or other baked goods: "I brushed the crumbs off the table."
* A small amount of something: "He only got crumbs of information."
* An expression of surprise or dismay: "Crumbs! I forgot to set the alarm!"
If you're looking for a more formal or specific synonym, you could use:
* Breadcrumbs: For the small pieces of bread specifically
* Bits and pieces: For a more general term for small pieces of anything
* Remnants: To describe something that is left over or remaining
* Fragments: For a more formal term for small pieces or parts
However, "crumbs" itself remains a perfectly acceptable and commonly used word in modern English.