These are another type of Electrolytic capacitors whose anode is made up of tantalum on which a very thin insulating oxide layer is formed. This layer acts as a dielectric and the electrolyte acts as a cathode which covers the surface of oxide layer.
The following figure shows how tantalum capacitors look like.
Tantalum provides high permittivity dielectric layer. Tantalum has high capacitance per volume and lower weight. But these ones are costlier than Aluminum Electrolytic capacitors, due to the frequent unavailability of tantalum.