Equities
By investing in equities, you are purchasing a slice of a company. Your fortunes become inextricably tied to that company, and you become a 'shareholder' in it. A publicly-traded company (one that issues stock for sale to shareholders in a public market) has a strong incentive to create 'shareholder value', because as the shares of stock become more valuable, they become more valued. When more investors compete for the same pool of shares, the price per share rises, making the company more valuable - a good outcome for both the company and all of the company's shareholders.
Equity investing broadly falls into three categories:
A) Individual stock investing where you buy a piece of the company.
B) Mutual fund investing where you give money to a manager who buys a 'basket' of stocks.
C) Index fund investing where you buy a whole sector of the stock market
This publication from the Securities and Exchange Commission (SEC) is a good place to begin learning:
Stock Investing
Mutual Fund Investing
Index Funds