"Verily, Allaah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allaah in worship, he has indeed invented a tremendous sin. "
"Verily! Allaah forgives not (the sin of) setting up partners in worship with Him, but He forgives whom he pleases sins other than that, and whoever sets up partners in worship with Allaah, has indeed strayed far away. "
It is reported that the Prophet (sallallaahu 'alayhi wasallam) said:
" The thing which I fear most for you is minor Shirk." Then he (sallallaahu 'alayhi wasallam) was asked about what is minor Shirk, and he said: "It is ar-riyaa (show-off)."
Reported on the authority of Ibn Mas'ood (radhiyAllaahu 'anhu) that the Messenger of Allaah (sallallaahu 'alayhi wasallam) said:
"Whoever died while supplicating another deity besides Allaah, will enter the Fire."
Allaah's Messenger (sallallaahu 'alayhi wasallam) said:
"Whoever meets Allaah, without associating partners with Him, will enter Paradise; and whoever meets Him as one who associated anything with Him, will enter the Fire."
Explanation by Allaamah 'Abdur-Rahmaan as-Sa'dee (rahimahullah):
Every instance of shirk in tahweed al-ilaahiyyah and tahweed al-'ibaadah negates tawheed .
And this shirk is of two types : (1) The blatant major shirk, (2) The subtle minor shirk.
Major shirk (shirk al-akbar) :
It is to make a rival to Allaah : that is called upon as Allaah is called upon, or to fear him, or hope in him, or love him, as Allaah is loved. Or to render a type of worship to him . This is shirk which does not leave its practitioner with any tawheed at all, this is the mushrik that Allaah has forbidden from Paradise, and his abode is the Fire.
It does not matter whether the worship rendered to other than Allaah is called worship or tawassul (seeking a means of nearness to Allaah), or even if it is given a name other than these. All of that is major shirk, because what is important is the reality of the thing and its implication, not the word or expression used to describe it.
Minor Shirk (shirk asghar) :
This is every saying or action which leads to shirk, like aggrandizing creatures in a way that does not quite reach the level of worship. (also) Like for example making an oath for other than Allaah, or riyaa (behaving for show).
Since shirk negates tawheed and necessitates eternal damnation in the Fire, and when it is the major type it make Paradise unlawful, and since bliss cannot be achieved except by escaping from it : then it is necessary for the worshipper : to duly fear it, to hurry to escape from it -- it's every path, means and cause -- and to ask Allaah to protect him from it, as was the habit of the prophets, the purified, and the best of creatures.
The worshipper must struggle to increase and strengthen the sincerity (ikhlaas) in his heart, and this is done by completely devoting oneself to Allaah -- being subjugated, repentant, fearful, hopeful, obedient, seeking His satisfaction and rewards in all that he does - without neglecting this in any matter, open or hidden.
True sincerity inherently repels both major and minor shirk, but whenever a type of shirk is present it will weaken one's sincerity.
AUTHOR: al-'Allaamah 'Abdurrahmaan as-Sa'dee (rahimahullaah)
SOURCE: 'Al-qawlu--s-sadeed fee maqaasidi at-Tawheeed, Sharh kitabu-tawheed'
 Qur'aan 4:48
 Qur'aan 4:116
 Saheeh at-Targheeb wat-Tarheeb (No. 29)
 Saheeh al-Bukhaaree
 Saheeh Muslim