A civil flag is a version of the national flag

National flag

A national flag is a flag that symbolizes a country. The flag is flown by the government, but usually can also be flown by citizens of the country.Both public and private buildings such as schools and courthouses may fly the national flag...

 that is flown by civilians on non-government installations or craft. The use of civil flags was more common in the past, in order to denote buildings or ships that were not manned by the military.

In some countries the civil flag is the same as the war flag

War flag

A war flag is a variant of a national flag for use by the nation's military forces on land. The nautical equivalent is a naval ensign — the battle ensign...

 or state flag

State flag

There are two separate meanings for the term state flag in vexillology – the flag of the government of a sovereign state, and the flag of an individual subnational state.-Government flag:...

, but without the coat of arms, such as in the case of Spain

Flag of Spain

The flag of Spain , as it is defined in the Spanish Constitution of 1978, consists of three horizontal stripes: red, yellow and red, the yellow stripe being twice the size of each red stripe...

 and Serbia

Flag of Serbia

The flag of Serbia is a tricolour consisting of three equal horizontal bands, red on the top, blue in the middle and white on the bottom. The same tricolour, in some variations, was the flag of Serbia throughout its history, and is the ethnic flag of the Serbian people.The state flag bears the...

, and in others it is an alteration of the war flag.
In Scandinavia, state and war flags can be double and triple-tailed variants of the Nordic Cross flag

Nordic Cross Flag

The Nordic Cross Flag, Nordic Cross, or Scandinavian Cross is a pattern of flags usually associated with the flags of the Scandinavian countries of which it originated. All of the Nordic countries except Greenland have adopted such flags...

. Many countries, particularly those with a British heritage, still have distinctive civil flags (technically civil ensign

Ensign

An ensign is a national flag when used at sea, in vexillology, or a distinguishing token, emblem, or badge, such as a symbol of office in heraldry...

s) for use at sea, many based on the British Red ensign

Red Ensign

The Red Ensign or "Red Duster" is a flag that originated in the early 17th century as a British ensign flown by the Royal Navy and later specifically by British merchantmen. The precise date of its first appearance is not known, but surviving receipts indicate that the Navy was paying to have such...

.