(1) See
packet filter,
filter bubble and
video filter.
(2) To select data. Filters use patterns (masks) against which data are compared and only matching data are "passed through," hence the concept of a filter. For example, email clients and servers can look for messages with text patterns that are recognized as spam (see
spam filter and
Bayesian filtering). An email filter can also be configured to send messages into specific mailboxes or forward mail to another address based on any matching criteria in the message. See
algorithmic filter.
(3) To change data. For example, a sort routine changes the sequence of data. A conversion routine (import or export filter) changes one type of data, text or graphics format into another. See
import filter,
image filter and
parse.