The words "an Act" and "a law" are often used loosely to be synonymous in an every day conversation, but "a law" is a broader term.
For example, the criminal code is "a law". But, while the original version of the criminal code may have been a single "act", the criminal code has almost certainly been amended by many other acts over the years after its original enactment as a single act.
An "act" is a single enacted bill proposed in a single legislative session approved in a single Presidential assent. A law, in contrast, can be the result of multiple acts approved in multiple Presidential assents at different times and then codified into a single statute.
Also, the term "a law" can be used correctly to refer to a particular holding of case law that is judicially created and not enacted by the legislature.
For example, someone might correctly say, "there is a law that makes it illegal to breach a contract without legal justification", even though that is a case law principle, rather than a legislatively enacted rule.
Confusingly, it isn't uncommon for the short title of a statute (i.e. a law) that is the product of multiple acts amending the original one, to be the short title of the original act that has been amended over the years.
For example, in the United States, the "1933 Securities Act" which was the short title of the original act giving rise to that statute, is still described by that short title, despite the fact that it has been amended scores of times since then well into the 21st century. (A careful writer would say "the 1933 Securities Act, as amended"). I suspect that this practice is also followed in India, because the American practice of naming statutes in this fashion dates back to English practice that was in place before the American revolution and has continued to be followed since then.
This isn't a hard and fast rule, however. Hence, another statute has the short title, "Statute of Frauds", even though it could have been described by the short title of the original act from the reign of Queen Anne.
But, in the abstract, "an act" has its more narrow technical meaning.
Incidentally, to prevent confusion, "the law" is a term much broader than either "an act" or "a law". The phrase "the law" encompasses not just a particular statute, but all statutes, all regulations that have the force of law, all treaties, the constitution, and all judicially created case law, not just statutes which come into being through the passage of bills as acts by the legislature.