The Normal Planet is a world similar to Earth as we know it, which is one of many planet types and, the only planet that is able to be colonized by any race at the start of the game in Galactic Civilizations III.
How to Identify?
Normal worlds can be identified by the appearance similar to that of Earth or Mars and, when hovering your mouse over the planet, it will indicate it's planet class with no restriction on colonization. Usually the lower class planets of 6 or below will have the appearance similar to that of Mars with a red or brown landscape. The other normal planets of class 7 or above normally have an appearance similar to that of Earth, with blue water and green/brown land.
Normal worlds are the main source of expansion and growth for most of the races at the beginning of the game. They can be colonized by any race with no restrictions right away and do not require any additional technology or abilities.
Because every race in the game is capable of colonizing these worlds at the start of the game, it is key to colonize the higher class worlds near your ZOC (Zone of Control [Influence]) as soon as possible filling in the lower class worlds in your ZOC as neighboring races start to infringe on your borders.
Which races able to immediately colonize?
Currently all the races are able to colonize normal planets at the start of the game. It is possible in the future that a race would be designed that would not be able to colonize these planets for one reason or another, through a DLC (Downloadable Content) or expansion.