Several terms are used with reference to Japanese maples and horticulture in general. Here are some, and what they mean:

Variety:  A variety is a tree that had no human interference in the plants' sexual reproduction. Totally natural occurrance.

Cultivar:  A tree that came about with human interference in the sexual reproduction process between trees. In other words, someone purposely tried to cross pollinate two trees to see what the resulting offspring might look like. The terms "cultivar" and "variety" tend to get used to mean the same thing but technically they are not. We don't fuss about this too much!

Seedling: A tree that has been grown from seed without any physical alteration such as in the grafting process. Seedlings are unique and beautiful trees in their own right. Seedlings of Acer palmatum are used as the rootstock for grafted trees. 

Grafted Tree: In order to obtain a perfect clone of a named tree (variety/cultivar) it must be grafted. That is, a cutting is taken from the tree of note and physically attached to a rootstock tree. When the union between cutting and rootstock successfully heals, the cutting becomes the new tree leader and the leader of the rootstock leader is removed. The genetics traits of the "new" tree are dominant.  

Dwarf: Generally, a tree that grows no more than approximately 6.5' (2m) tall.

Upright: Refers to the growth habit in an upright manner, as opposed to weeping.

Variegated: Term given to trees with more than one colour present on the leaves.

Reticulated: Term given to the leaves when a ghost-like, almost translucent appearance is seen. The veins are prominently seen. These trees almost always need protection from hot afternoon sun.

Dissectum (Laceleaf, Cutleaf): Reference to the lacy, deeply cut fine leaves of some types of maples. These maples almost always are weeping types. Acer palmatum 'Seiryu' and 'Lionheart' are two exceptions. Dissectums generally grow anywhere from 3'(2m) to 12'(3m) tall and wide, depending on the variety. Some are smaller and some are larger.