Beyond what is already said, Effective Communication has several other things/components in it.
According to A. Barbour, author of Louder Than Words: Nonverbal Communication, an astounding 93% of all communication is non-verbal
1. Speaking (verbal/words)
2. Posture and Body Language (gestures)
3. Eye Contact and Facial Expressions
4. Physical Appearance (dress, grooming, shoes, jewelry, body art etc.)
4. Tone of Voice and Pace of Words (voice modulation, and speed at which one talks)
5. Sentence Construction
6. Proper Use of Capitalization (All capitals in written English implies the writer is emotionally charged, shouting, angry etc.)
7. Proper Use of Grammar and Spellings (Is communication skill a must of HR professionals? If so, why? And how can one develop it?) or (Are communication skills required for HR professionals? If so, why? And how can one develop them?)
A simple Google Search on Communication (or Non-Verbal Communication) would give you a ton of results.
Enjoy your research :-)
Best,
-Som G