This series include all elements in the sub-series Lanthanides and Actinides of the inner-transition elements and at least part of the sub-series Transactinides, which are the elements following the Actinides series. In general these elements are known for their hardness, high density, high melting point and boiling point and heat conduction although there are exceptions. 

These elements all have a d electron as the differentiating electron in their electron configuration. In other words, their outer most electrons are always in the d orbital.

(Fe), for instance, has an electron configuration as follows:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Note that the 4 S orbital actually filled before 3d. The 3d acts as the outer most orbital and is the one involved in reactions.

These metals frequently have more than one oxidation state. Our sample Iron has two oxidation states +2 and +3. These may be represented as Iron II and Iron III or the ferrous ion (+2) and ferric ion (+3). Ferrous and ferric are Latin terms. The symbols for iron II and iron III are Fe II and Fe III as one might expect.

Many of these elements are electropositive enough to replace hydrogen (H) from acid solutions. They also form complex ions and coordinate covalent compounds (in which one element dominates both electrons).

The ions of transition elements tend to be colored, making them easy detect in quantitative analysis. The copper(Cu) ion, for instance is aquamarine and cobalt (Co) is blue.