In short: speculation is buying something only because you expect its price to rise and someone else to pay more — regardless of what stands behind the asset.
Simply put: An investment rests on something the asset produces: profit, a dividend, rent, a right. Speculation rests only on the expectation. It can happen with a share too, when you buy it "because it is rising". If there is no value of its own behind the price, there is nothing for it to return to when the expectation fades.
Example: Vanya buys an asset for €100 because "it doubled in a month". She does not know what it produces. If buyers stop coming, the price may fall to €30, and there is no income to hold it up.
Watch out: ask: "What do I get apart from the hope that the price will rise?" If the answer is "nothing", it is speculation.
Source: Benjamin Graham, The Intelligent Investor (Harper & Brothers, 1949) — the distinction between investment and speculation