What is the difference between a Clustered and Non-Clustered Index?

Clustered index sorts data in a table saving data as leafs of the index tree. Non-clustered index does not sort data - it has references to the location of data.

Also, A Table can have 1 Clustered Index and 999 Non-Clustered Indexes.

Clustered index, basically can be applied only on one column i.e. on primary key column i.e. Table can have one and only clustered index. It’s faster than non clustered index.It decided the order of the data in table.
Non clustered index, it can be applied on more than single column of the table. It is slower than clustered index as it requires more memory for operations on table. It defines data in index only.