Dot Plot.

A black square followed diagonally by one or more red squares indicates matching strings in both sequences. These need not occur on the principle diagonal. A string at the end of one sequence may match a string at the start of the other. Just to convince you that long matching strings are unlikely to occur by chance, on the next page I've put the same program, but now both sequences are of length 20 with randomly chosen residues.
But before we do that, press the more button and input the strings "ASDFGHJKL" and "ASDFTJKL". This will give you an idea of what to look for when you have a string that's a little broken, but almost intact.