Not properly citing where your information came from is considered plagiarism. You are guilty of plagiarism if you do not give credit with direct quotes as well. Some writers will even go so far as to take direct statistics and resources and take credit for the information. Giving completely false references is also plagiarism.
When you take several different references, change the verbs and use synonyms but the format is someone else's, that is considered plagiarism. There is no research here, just the regurgitating of another writer's work. All you are doing is changing a word here or there to pass it off as an original work of your own.
Trying to pass off another person's research and work as your own without making any changes to the thoughts, flow, and language and citing the exact same resources is called "ghost writing" and is blatant plagiarism. This type of plagiarism should really make a copy editor question the writer's character in trying to present this type of work.
As a writer, you may feel that if it is your work to begin with, you can use it whenever and however you want. With this type of plagiarism, writers may take another piece of their work and basically re-submit it to another publisher. In other cases, a writer may take the same complete thought they have and just rewrite it using different vocabulary. Both of these are considered to be plagiarism.