What is the difference between intials cells and meristems cells?
Meristems are undifferentiated plant cells. There are two types: 


apical mesistem that occurs at tips or branches and roots as well as buds which produces elongation, and
lateral meristem or cambium that produces lateral growth (stem thickening).


As cambium cells divide, cells toward the inside differentiate into xylem (wood) - these cells are 'xylem initials'. Cells toward the outside that eventually differentiate into phloem tissues (inner bark) and are 'phloem initials'.

One could similarly label apical meristem cells that are in the postion and/or process differentiating into leaves or flower organs as x-initials.