"Orange Dwarf Star" is a colloquialism for stellar objects that are, more properly, called "K-type main-sequence stars" and are designated K V. The first is a letter, the second a Roman numeral. These stars are a sort of intermediary between yellow and red dwarves, hence the colour reference.
Their sizes are less than that of our Sun, ranging from just a little smaller to half its size. They tend to remain main-sequence stars much longer than, say, yellow dwarves (like our Sun). Whereas yellow dwarves' life spans may be around some 10 billion years, orange dwarf stars can remain main-sequence for up to three times as long. Because they're around for a lot longer, they outnumber yellow dwarves considerably. Also, their age makes them more likely to have a developed planetary system.
This is denoted by the "K" in the official designation. It simply depicts the temperature of the surface of the star. K-type stars have surface temperatures ranging from 3,900 to 5,200 degrees Kelvin.
The spectral type can be further subdivided into grades of hotness, represented by the numbers zero (the hottest) to nine (the coolest).
The Roman numeral "V" represents the orange dwarf as being main-sequence, i.e. fusing hydrogen into helium.
Certain stars have been designated as representing the various degrees of hotness: