Academic question Clustered Index or Nonclustered Index

Hi guys, I'm getting a little bit confused about clustered and not clustered index. For example:

create table forum (xxxxx int identity) 

it's a clustered index or not (I think don't)?

create table forum (xxxxx int primary key on xxxxxx)

In this case I suppose that auomatically SQL create a clustered.

For your experience did you find really usefull to manage clustered and non clustered index? And when in first case and when in the second case?

Thank you a lot, it's only just for sharing some experience (bear in mind I'm data analyst and not developer...but I started to study all the features of SQL)