Flag, ensign, standard, banner, color, streamer, pennant, pendant, pennon, jack are not always clearly distinguished.
Flag, the comprehensive term, is applied to a piece of cloth that typically is rectangular, is attached to a staff, mast, halyard, or line, and carries an arrangement of colors, an emblematic figure, or a motto. The purpose of a flag is primarily to serve as a sign or symbol of a nation, a branch of the service, an organization, or an office, but it may also serve as a signal (as in military or naval operations) or in giving information (as of a weather change or the approach of a train).
Ensign is applied chiefly to a flag that indicates nationality and specifically to one flown by ships at sea.
Standard and banner are more or less literary terms for the flag (as of a country, a party, or a religious, civic, or patriotic organization) thought of as a rallying point or as something to be followed.
Standard especially suggests the former because the term originally designated and still often designates a flag or a sculptured figure raised on a pole so as to be a gathering point for all who belong under it.
Banner basically applies to a flag (often hung downward from a crosspiece instead of flying from a staff) of an individual (as an emperor, king, lord, or military leader) which was formerly flown from windows or doors or carried aloft at the head of a procession (as of troops marching to war).
Color (most frequently found in the plural colors ) may apply to a national flag, to a flag emblematic of affiliation or partisanship, or to a flag of most military units; the term is particularly likely to suggest military activity or display.
The remaining terms are highly specific and definite in implications.
Streamer applies to a long narrow flag (as on the masthead of a government ship) that floats in the wind; pennant and pendant, the latter more English than American, apply to a streamer that is long, narrow, and tapering. Pennant even more often applies to a narrow flag, typically triangular, which is flown by ships, which is used in signaling and in decorating, or which is exhibited (as by a baseball club) as a sign of championship.
Pennon may apply to a narrower flag or a small streamer suitable for attaching to a lance. Jack denotes a small oblong flag indicating nationality which is hoisted on a staff at the bow or bowsprit cap of a ship or one used in signaling.