I'll attempt to describe this without using any biological references (therefore, highly oversimplified).

DNA is made up of long strands (sequences) of just four building blocks (given the initials A, C, G and T). Here's an example sequence:

A
|
C
|
G
|
T
|
G
[download]

The sequence here is ACGTG; the reverse sequence would be that read from the other end, i.e. GTGCA.

DNA strands form pairs. Each of the building blocks pairs with only one of the other building blocks: A with T and C with G (and vice versa). Knowing the sequence of one strand, the complementary sequence is easily determined:

A==T
|  |
C==G
|  |
G==C
|  |
T==A
|  |
G==C
[download]

So, the complement of ACGTG is TGCAC; and the reverse complement is just that read from the other end, i.e. CACGT.

As I said, that's highly oversimplified but hopefully is enough to explain the terms.