Who is Brahm - ब्रह्म कौन है? (Kshar Purush / क्षर पुरुष or Kaal / काल)
Brahm (ब्रह्म) is the master of 21 brahmands. He is also known as Kaal (काल), Jyoti Niranjan (ज्योति निरंजन) or Kshar Purush (क्षर पुरुष).
God Brahm (ब्रह्म) is the father of Brahma (ब्रह्मा), Vishnu (विष्णु) and Shiva (शिव). He is also addressed as Mahabrahma (मंहब्रह्मा), Mahavishnu (मंहविष्णु), and Mahashiva (मंहशिव). To have complete knowledge about the status of Brahm (ब्रह्म), the readers need to be aware of Creation of nature.
Brahm (ब्रह्म) - in Shrimad Bhagavad Gita
Brahm (ब्रह्म) / Kaal (काल) is the speaker of Shrimad Bhagavad Gita. In Shrimad Bhagavad Gita, Brahm (ब्रह्म) has been addressed as the "Unmanifest" God. In Gita Adhyay 11, Shlok 32, this unmanifest Brahm (ब्रह्म) himself claims to be Kaal (काल). Moreover he says that I have "appeared" now. He then shows his "Viraat" (विराट) form to Arjuna which he clarifies further by saying that this form which I am showing you now has never been seen by anyone before and this form of mine cannot be witnessed by anyone by doing any form of yagyas (यज्ञ), meditation (तप) or any other effort. Please read Who gave the knowledge of Bhagavad Gita for further clarification.
Brahm (ब्रह्म) - in Purans
In Brahma Puran (ब्रह्मा पुराण), Vishnu Puran (विष्णु पुराण), and Shiv Puran (शिव पुराण), this Brahm (ब्रह्म) is addressed as Mahabrahma (मंहब्रह्मा), Mahavishnu (मंहविष्णु), and Mahashiva (मंहशिव) respectively. People get confused when reading the Purans as each puran describes the origin of one God from another. For example in Vishnu Puran, the origin of Brahma and Shiva has been described from Vishnu. Likewise in Shiv Puran, the birth of Vishnu and Brahma has been described from Shiv. In reality all three Brahma, Vishnu and Shiva have originated from Brahm (ब्रह्म) or Kaal. Brahm (ब्रह्म), Kaal is the father of the three (trimurti - त्रिमूर्ति).