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

Variety: Similar to humans, the offspring of Japanese maples are not 100% exact copies of the parents. Occasionally, a Japanese maple will come about from natural circumstances that is so unique that it happened to catch someone's attention. And consequently, after years of observation to confirm the uniqueness, the tree may be introduced and sold in the nursery trade. 

Cultivar:  Similar to a variety but specific human intervention in the breeding process helps to create a new "variety" called a cultivar. The words, variety and cultivar are often used to mean the same thing. The difference is, a variety came about through natural, environmental breeding while a cultivar had human help. 

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 are also 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 cutting successfully heals, the cutting becomes the new tree leader and the leader of the rootstock is removed.

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 in variegated types. These trees often need protection from hot afternoon sun.

Dissectum (Laceleaf): 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.