"And when We change a Verse [of the Quran, i.e. (abrogate) its order] in place of another, and Allah knows the best of what He sends down, they (the disbelievers) say: "You are but a Muftari! (forger, liar)."
Nay, but most of them know not. Say, Ruh ul-Qudus [Angel Gabriel) has brought it (the Quran) down from your Lord with truth, that it may make firm and strengthen (Faith) of those who believe and as a guidance and glad tidings to those who have submitted (to Allâh as Muslims)"
[16: 101-102]

As for al-Naskh in the Qur'an, it can be in any of the following modes:
1) To cancel a ruling and keep the text for reading (preference), such as: (1) the verse of the will on behalf relatives [2: 180],

(2) Surah al-Baqarah: 240, and (3) the verse of easing the task of fighting [8: 65].

2) To cancel a ruling and the text as well, like: "It was among the recitation (of the Qur'an): ten known times of suckling. This was about ruling of a child's suckling"
 [Reported by Imams al-Bukhari and Muslim ]).

3) To cancel only the text not the ruling, such as the verse concerning stoning the married one who commits Zina. It was as fallows: The married man and the married woman should be stoned if they commit Zina.
al-Naskh is only related to orders and prohibitions not the tidings.

As for number of al-Naskh, al-Suyuti in his book al-Itqaan said, 'twenty-one verses in the Qur'an were abrogated; some were agreed upon, while others are not. These abrogated verses are in the following Surahs: Al-Baqarah, Al-'Imran, An-Nisa', Al-Ma'idah, Al-Anfal, At-Taubah, An-Nur, Al-Ahzab, Al-Mujadilah, Al-Mumtahinah and Al-Muzzammil."

As for wisdom behind al-Naskh, scholars of Islam said: 'change is the nature of existence and beings and abrogation is one of the Divine ways to address these changes. If al-Naskh is reasonable to apply to beings, then it should not be considered as strange concerning rulings about them since the Ummah is still developing and advancing towards what is better. In addition, Allah is Able to do everything and He is the Only Owner of this universe and He can do what He Wills. {He cannot be questioned as to what He does, while they will be questioned.} [21: 23].

Thus, al-Naskh constitutes a step towards what is better and superior. Al-Naskh is often a means of bringing ease and relief, since Muslims were required to follow what is higher, stronger and better in the first stage of education and building the Ummah. Thus, it was obligatory to perform Qiyam al-Lail (prayer at night) or that 20 Muslims were to face 200 of the enemy.

Then, al-Naskh of those rulings constituted a form of relief, of soothing, and cancellation of hardship. Another wisdom behind al-Naskh is to bring obedience to the Divine Orders.

Here, we state that abrogated rulings should not be put into practice, since they were revealed for some fixed period of time. For example, it is not allowed to put a doer of Zina in jail since such a ruling was abrogated. Another example is that it is not allowed for a woman whose husband has died to enter into Iddah (awaiting period) as stated in Surah 2: verse 240 since it is abrogated by verse 2. 234.

Finally, al-Naskh was done only by revelation (ordered by Allah) and  was totally completed in the lifetime of Muhammad, peace be upon him.
It is not acceptable to consider abrogation to revelation (Quran or Sunnah of Muhammad, peace be upon him) has been or could ever be, accomplished through consensus or any analogy.

Allahu 'Alim (Allah is the All Knower)

Salam alaykum, and be sure to pray for me (dua), inshallah.
Yusuf Estes 

 [Allah uses the words "We", "Us" and "Our" throughout the Quran to indicate His Majesty and Authority, also known as the "Royal - WE"]

"And when We change a Verse [of the Quran, i.e. (abrogate) its order] in place of another, and Allah knows the best of what He sends down, they (the disbelievers) say: "You are but a Muftari! (forger, liar)."
Nay, but most of them know not. Say, Ruh ul-Qudus [Angel Gabriel) has brought it (the Quran) down from your Lord with truth, that it may make firm and strengthen (Faith) of those who believe and as a guidance and glad tidings to those who have submitted (to Allâh as Muslims)"

More on "Abrogation" in Shar'iah:

The two terms, an-Naasikh and al-Mansoukh, are from the principlles of Shari'ah (Islamic Law).

"Naskh" means abrogation of a ruling coming with a new and recent text, in such a way, if the latter had not been revealed, the former rule would still be in place and would apply.

"Naskh" has three pillars:

1-     An-Naasikh is Allaah and His Prophet  in things in which Allaah permitted him to change. Therefore, it is not possible for anyone to claim that a religious ruling has been abrogated except with evidence from the Book of Allaah and the Sunnah of the Prophet .

2-     Al-Mansookh 'Anhu is the previous ruling.   

3-     Al-Mansookh Ilayhi is the new ruling;

Here is an example of "abrogation" from Allah:
From the beginning in Islam, before the month of Ramadan was an obligation for fasting, the day of "Ashuraa'" was a obligatory.
Then Allah "abrogated" this obligation with the commandment for fasting the entire month of Ramadan. (see: Chapter 2, verse 183)
So "An-Nasikh" is Allah, and "Al-Mansoukh Anhu" is the obligation for fasting the one day of Ashuraa, and "Al-Mansoukh ilayh" is the fasting of the month of Ramadan.

And Allah is the All-Knower - Allahu 'Alim

Note: This is based on fatwahs (Islamic Rulings) found in English on www.IslamWeb.net
Another reference: www.IslamQA.com or www.JustAskIslam.com