The following are some of the difference between Islam and Qadianism (Ahmadiyyat )

1. Qadianism (Ahmadiyyat) is based on the belief that Mirza was an
improved second reincarnation of hazrat Muhammad (SAW) .
2. Qadianism (Ahmadiyyat) rejects the concept of absolute Finality of
Prophethood in hazrat Muhammad (SAW ), as confirmed in Quran,
Hadith, Sunnah of the Holy Prophet(SAW) , Tradition of Companions,
the writings of Muslim Scholars and personalities, and concensus of
the entire Ummah for almost 1500 years.
3. Qadianism (Ahmadiyyat) maintains that Mirza Ghulam Qadiani was a Prophet ( nabi and rasul) of God.
4. Qadianism (Ahmadiyyat) rejects the concept of completion of the
revelations of Allah(SWT) in the Holy Quran.
5. Qadianism (Ahmadiyyat) holds that Mirza Ghulam Qadiani's
revelations (Books) were on the same level as all prior revelations
(Quran, Bible, Torah). In their view, simply following Quran and
Sunnah, as Muslims have done since the beginning of Islam, is not a
basis for living a righteous life and gaining the pleasure of the
Creator. Interestingly, the Qadiani leadership has refused to allow a
translation of these books, so that everyone may become familiar
with the irrational teachings and contradictory claims of the founder
of their organization.
6. Qadianism (Ahmadiyyat) rejects authentic Hadith based on Mirza's
alleged revelations and teaches his personal interpretation of the
Holy Quran . Qadiani (Ahmadiyya) leadership has forged several
unauthentic translations of the Holy Quran to try to confuse and
mislead uninformed individuals.
7. Qadianism (Ahmadiyyat) teaches that Jesus ( pbuh) had been
crucified, but did not die from his injuries. Instead, it advocates the
view that Jesus( pbuh) recovered from his injuries, escaped to
Kashmir (India), where he lived for another 86 years, and is buried
there.
8. Whereas Jesus (pbuh ) is acknowledged as a great prophet of Allah in
Islam, Mirza Ghulam took the liberty of making demeaning and vulgar
remarks against him and his honored mother, rejected his miracles,
belittled his mission and denied his return before the Day of
Judgment. It is such unbecoming teachings that have resulted in
hateful retributions by extremist Christians evangelical groups
against Muslims, Prophet Muhammad(SAW), and Islam.
9. Qadianism (Ahmadiyyat) maintains that Mirza Ghulam Qadiani was
"the promised Messiah". The Qadianis (Ahmadis) reject the advent of
Jesus Christ (pbuh) , son of Mary, as the Messiah, just before the Day
of Judgment.
10. Qadianism (Ahmadiyyat) teaches that Mirza Ghulam Qadiani was also the promised Mahdi (guided one).