A DSLfilter
has two phone line connections, with a filter in the middle that
removes the frequencies used by DSL. Plugging it "in-line" with a
telephone device will allow the telephone device to operate properly
without either interfering or being interfered with by the DSL signal.
A DSL splitter
has the same characteristics as the filter, except that it has a
"pass-through" port that brings the original phone line, with the DSL
signal, around the filter for further connections. This makes it easier
to install in situations where you are trying to "split" the telephone
line into two parts. So that one can go to the phones, and the other
can go to the DSL modem.