Lets answer your question as you asked
create clustered index ix_index1 on table1(col1)
create clustered index ix_index2 on table1(col2)
You are creating a clustered index(CI) but as soon as you create CI on col1 you cannnot do it on col2 as on a table you can have only one CI. A CI is physically a table arranged in particular order. Since a table cannot be arranged according to two order you can only have one CI on a table. Assuming Col1 satisfies all requirements to be candidate for Clustered index uniqueness and no null.
create clustered index ix_index1 on table1(col1, col2)
You are creating clustered index on two col(col1 and col2 ). This can be because single col does not have unique record but combination of col1 and col2 have unique records. A kind of composite clustered index.
create nonclustered index ix_index1 on table1(col1)
create nonclustered index ix_index2 on table1(col2)
You are creating non clustered index on col1 and then on col2. You can have multiple non clustered index.But only one clustered index on a table.
create nonclustered index ix_index1 on table1(col1, col2)
You arecreating Non clustered index on two columns combined.
This was specific reply to your questions. I would like you to read MSDN resource on indexe ssuggested by Ron. you can also refer to below link by Brent Ozar