Explanation from Sheikh Ibn Baz
Shaykh Ibn Baz (may Allah have mercy on him) said in Majmoo' al-Fataawa, 3/423-424:
What is mustahabb (recommended) is to beat on the daff at weddings. This is mustahabb for women only, in order to announce the wedding and to distinguish it from fornication. There is nothing wrong with women singing amongst themselves, accompanied by the daff, so long as the songs contain no words that encourage evil or distract people from their duties. It is also a condition that this should take place among women only, and there should be no mixing with men. It should also not cause any annoyance or disturbance to neighbours. What some people do, of amplifying such singing with loudspeakers is evil, because of the disturbance it causes to other Muslims, neighbours and others. It is not permissible for women, in weddings or on other occasions, to use any instrument other than the daff, such as the oud, violin, rebab (stringed instrument) and so on. This is evil, and the only concession that women are given is that they may use the daff.Use of a Simple Drum
Use of the "duff" (simple drum) is permitted in Islam for a practical purpose (such as establishing a simple beat for exercise, rowing, and other labors), but not to be done by a Muslim for entertainment or idleness. In such instances, it must not be played as music, such as by enhancing rhythm, whistling a tune and whatnot. Ibn al-Qayyim said in Ighaathat al-Lahfaan (1/256) that the prohibition against the use of instruments refers to "all kinds of things used for entertainment."
According to the Sunnah, females can sing and beat the duff on the two 'Eids (specific Muslim celebrations) and to announce a Muslim wedding amongst themselves, and their voices shouldn't be raised loud enough or near enough to be heard by the men.
Poetry is also permitted, but it must not contain shirk (the major sin of associating a partner in worship with Allah, i.e. trinity, major or minor shirk, etc.).
The woman's voice should not be used in an attractive way around non-mahram men and to them it must be limited to necessity (physical necessity such as asking for something at the store, not emotional 'necessity' such as social conversation). Men and women must not engage in idle or social conversations with non-mahrams (those not related as specified in The Quran). Muslim men should avoid listening to a non-related (i.e. non-mahram) female voice (such as pop singers, etc.) except when it is unavoidable or due to her speaking out of necessity and not socializing. .
Salamu Alekum jazakuAllahu khairan sis