For example, if there are 100 million shares of company XYZ and those shares are trading at $35 a share, then company XYZ has a market capitalization of $3.5 billion.
Stocks with a larger market capitalization are generally seen as 'safer' investments, since the company itself is larger. However, large-cap stocks also do not have as much growth potential as small cap stocks. For a $1 billion stock to triple, it just needs to become a $3 billion company. For a $200 billion company to triple, it needs to become a $600 billion company.
Stocks are generally classified as mega-cap, large-cap, mid-cap, small-cap, or micro-cap stock based on their market capitalization. There is no set definition for these terms. What some consider a small-cap others might consider a micro-cap. Nevertheless, here's a general guideline to give you an idea what market capitalizations fit these terms:
Mega Cap: $200 billion or more (very few companies fit this category)
Large Cap: $10 billion to $200 billion
Mid Cap: $2 billion to $10 billion
Small Cap: $250 million to $2 billion
Micro Cap: Less than $250 million