Decode: using for reporting purpose. and it is implemented
by if stmt.
Ex: select decode(job,'CLERK','executive',job) from emp;

Case : implemented by if & elsif condition. it is using
pl/sql equality conditionality.
Ex : select  case job='CLERK' then 'executive' else 'no' end
a from emp;

it is also used for multipul colms & multi conditions.

in above stmt a is reffered alias name.

Decode and Case functionwill work in the same way.the only 
difference is we can write conditions in side the case 
function.
for Ex;
case(if c>d,d...)
decode(job,'clerk'...)

Decode
1.It’s a function
2.can compare only discrete vaules
3.Pl/Sql standards
4.cannot process null 

Case
1.It's an Expression
2.can hande range values
3.ANSI Standards
4.Processing time is faster when compared to Decode
5.can process null

Decode
The default operator for comparison is '=',
for e.g select deptno,decode
(deptno,10,'dept10',20,dept20,default)

so if the deptno=10 then the value is replaced by dept10 
and so on,

whereas in Case:

we can use any operator other than '=',
for e.g 
select deptno,
case when deptno=10 and sal>1000 then 'dept10'
else
'dept20'

Hope the difference is made clear.

Regards,
Judith