Waheguru is not a doctrinal term but a term of address and of remembrance, and it is the word Sikhs actually say. Its etymology is not settled: it joins an exclamation of wonder, wah, to guru, and it is rendered "Wonderful Lord", "Wondrous Enlightener" and "Praise to the Guru" among others. It appears in the Guru Granth Sahib chiefly in the swayyas of the Bhatts, the bards whose panegyrics close the scripture, and it became the gurmantar, the word given to a Sikh at initiation and repeated thereafter. Akal Purakh, the Timeless Being, is the term that does the theological work in Sikh writing; Ik Onkar is where the argument starts; Waheguru is what a person says.
The central practice is naam simran, remembrance of the Name: repeating it, holding it in mind, and singing the scripture as kirtan, which is why so much of Sikh worship is musical and why the Guru Granth Sahib is arranged by raga rather than by subject. The Mul Mantar, Japji Sahib, Jaap Sahib, Rehras Sahib, Kirtan Sohila and the Ardas are all, in the end, forms of this. The mantras and hymns in this atlas that now point at this page point at it because they address it in the second person or name it directly, not because they are Sikh in origin.
One distinction is worth stating because it is very often got wrong. The Guru Granth Sahib is the eternal Guru of the Sikhs, installed as such by Guru Gobind Singh in 1708, and it is treated with the honours due to a living Guru: it is enthroned, attended, and put to rest at night. It is not God, it is not worshipped as God, and the distinction between honouring the Guru and worshipping the one Reality the Guru points to is one Sikhs make carefully. An observer who describes the reverence given to the scripture as worship of a book has misdescribed it.