TSQL - Dense rank vs rank vs row number

select

       T1.Field1

       , DENSE_RANK() over( order by Field1 asc ) as DenseRank

       , RANK() over( order by Field1 asc ) as DenseRank

       , ROW_NUMBER() over( order by Field1 asc ) as RowNumber

from

       (

              select 'a' as Field1

              union all select 'a'

              union all select 'a'

              union all select 'b'

              union all select 'b'

              union all select 'c'

              union all select 'd'

       ) as T1

;