A constructed language is a language whose phonology, grammar, and vocabulary, instead of having developed naturally, are consciously devised or invented as a work of fiction. Constructed languages may also be referred to as artificial languages, planned languages or invented languages and in some cases, fictional languages.
What a constructed language should be Edit
The first thing to focus in a constructed language shall be the fundament, the base. Endings, the buildup (Shall it be SVO, SOV, VOS or something completely different?), verbs, adverbs, vowels, sounds, nouns and adjectives.
Once you’ve sufficiently built that up, you shall begin slowly on vocabulary. Remember grammar rules and rules of language. Which alphabet shall be used? That is up to you! Build the vocabulary at last, and then you are pretty much done. Remember to learn the language to yourself!