boundary value: 
for int suppose boundary is say 1 to 50
then test for 0,2 values & 49, 51 vales.

for string... test for null or length or capacity of
string(size).

equivalence partitioning: 
e.g.: divide(a,b) ...boundary -50 to +50
(do not forgot zero for 'b')
test for... -51, and -1 --------(1)
also 1, and +51 ------------(2)
(1),(2) are two partition. 

1000-2500----->no tax
2501-4000---->tax 5% of there Salary.
In EQUIVALENCE : values from 0 t0 999 is put together in 
one class c1-means it shd behave in same manner- invalid 
values
values from  1000 to 2500 in class c2 - notax
values from 2501 to 4000 in class c3- 5% of there salary
like wise grouping the values in bifferent classes,
To put this in simpler words, since it is practically 
infeasible to do exhaustive testing, the next best 
alternative is to check whether the program extends similar 
behaviour or treatment to a certain group of inputs. If 
such a group of values can be found in the input domain 
treat them together as one equivalent class and test one 
representative from this. This can be explained with the 
above  example. 

Boundary Value: taking the above example
1. if user typedin 999rs -it shd promt as invalid
2. when 1000rs it shd not promt as invalid and no tax shd 
be taken.
3. when 2501 is typed 5% of tax shd be taken

this is called Boundary value