External Hijab is a phrase used to denote garments (typically female) associated with the modest dress of Muslims. It is merely one aspect of hijab which is an all encompassing "modesty" that every Muslim is called to have.
Glossary of External Forms of Hijab for Women
Abaya
A type of outergarment that covers the head and body; it is all but shapeless
A veil that is tied on at the forehead and falls to cover the entire face but has no cut-out for the eyes; instead, the fabric is sheer enough to be seen through
Also known as the Afghan Burqa. Covers the entire body and has a grille over the face that the woman looks through. May have slits for the hands
Chador
A type of outergarment that covers the head and body; it is sort of like Khimar (2) - see below - but comes down to the ankles or the ground. May have slits for the hands
Dupatta
Also called "shayla" and "milfeh"; a type of headcovering that is wrapped around the head
Hijab (1)
generic
The entire modest dress of the Muslim woman
Hijab (2)
generic
The headscarf; this is properly referred to as a "khimar"
A type of headcovering that is a square of fabric folded into a triangle then placed over the head and fastened under the chin; this is probably the most common current style, especially in Western countries. See explanation in the article on Hijab
Jilbab (1)
generic
The term used in the Quran (Surah al-Ahzab ayah 59) to refer to the outergarment
The word "khimar" is most commonly used to refer to a circular type of headcovering with a hole cut out for the face, which usually comes down to the waist. Note the variations Buknuk and Chador above, which are the same style but different lengths
Milfeh
see above
See Dupatta
Niqab (1)
generic
Any type of veil for the face or outfit that covers the face
The name typically used to denote the clothing worn by a pilgrim during either the Hajj or Umra. For a male the first part is the ezaar, a piece of cloth wrapped around to cover from the ankles to the abdomen. The second piece called the reda is draped over the shoulders to cover the upper body. The cloth is to be plain, white, and unsewn. For women typical and unpretentious clothes will be their Ihram.