Short answer
Muslims believe he is the final messenger of Allah, sent to all people. Through him came the Quran, and his Sunnah (words and example) shows how to live it. Muslims love him more than their own families, follow his example, and send blessings on him, but they worship only Allah.
Sunni belief holds that belief in his prophethood is part of the shahada, that obeying him is obeying Allah, that his Sunnah is the second source of Islam, and that complete faith requires loving him above everyone else. He is a human servant of Allah, not divine.
The four schools agree on this.
Explanation
The final messenger. The Quran calls him "the Messenger of Allah and the seal of the prophets" (33:40). No prophet comes after him, and his message is for everyone. That is why the testimony of faith has two parts: there is no god but Allah, and Muhammad is the Messenger of Allah.
The one who brought and explained the Quran. The Quran was revealed to him over 23 years. His Sunnah shows how to pray, fast, give zakat and perform Hajj, and how to be a husband, father, neighbour and leader. Allah says that in him we have "an excellent example" (33:21), and that whoever obeys the Messenger has obeyed Allah (4:80).
Loved above all. He said none of you truly believes until he is dearer to him than his father, his child and all people. Muslims show this by following his Sunnah, learning his life, defending his honour and sending salawat on him. See What are the benefits of sending salawat?
What he taught. In short: worship Allah alone, believe in the Last Day, pray, give, keep ties of kinship, speak the truth, be just and merciful, and treat people well. Muslims honour him but never worship him; see Do Muslims pray to Muhammad? His life is on Prophet Muhammad ﷺ and Seerah.
The evidence
Quran, Surah Al-Ahzaab 33:40
مَّا كَانَ مُحَمَّدٌ أَبَآ أَحَدٍۢ مِّن رِّجَالِكُمْ وَلَٰكِن رَّسُولَ ٱللَّهِ وَخَاتَمَ ٱلنَّبِيِّۦنَ ۗ وَكَانَ ٱللَّهُ بِكُلِّ شَىْءٍ عَلِيمًۭا ﴿٤٠﴾
Muhammad is not the father of [any] one of your men, but [he is] the Messenger of Allah and last of the prophets. And ever is Allah, of all things, Knowing.
He is the final prophet.
Quran, Surah Al-Ahzaab 33:21
لَّقَدْ كَانَ لَكُمْ فِى رَسُولِ ٱللَّهِ أُسْوَةٌ حَسَنَةٌۭ لِّمَن كَانَ يَرْجُوا۟ ٱللَّهَ وَٱلْيَوْمَ ٱلْءَاخِرَ وَذَكَرَ ٱللَّهَ كَثِيرًۭا ﴿٢١﴾
There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and [who] remembers Allah often.
He is the model for believers.
Quran, Surah Aal-i-Imraan 3:31
قُلْ إِن كُنتُمْ تُحِبُّونَ ٱللَّهَ فَٱتَّبِعُونِى يُحْبِبْكُمُ ٱللَّهُ وَيَغْفِرْ لَكُمْ ذُنُوبَكُمْ ۗ وَٱللَّهُ غَفُورٌۭ رَّحِيمٌۭ ﴿٣١﴾
Say, [O Muhammad], "If you should love Allah, then follow me, [so] Allah will love you and forgive you your sins. And Allah is Forgiving and Merciful."
Following him is the sign of loving Allah.
Sahih al-Bukhari 15
Sahih... لا يؤمن أحدكم حتى أكون أحب إليه من والده وولده ...
"None of you believes until I am more beloved to him than his father and his child."
Love for him is part of faith.
Where the ruling is recorded
- al-Shifa (Qadi Iyad), Part on the duty of loving, obeying and honouring him. The classical work on his rights over the ummah.
- al-Aqida al-Tahawiyya, Section on the prophethood of Muhammad. He is the chosen servant, the final prophet, sent to all jinn and mankind.
- Zad al-Ma'ad (Ibn al-Qayyim), Introduction on the need to know his guidance. Explains why knowing his way is necessary for every Muslim.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.