Explanation:
For a given country, you can find the actual breakdown of its government’s revenues in the Statement of Government Operations (p. 38, IMF). This is like the government’s income statement. Some governments might give it different names and some governments might be more transparent than others.
The other answers to this question give some really good examples that I would group into five broad categories of government revenues.
Taxes. This is usually the lions share of most governments revenue. All kinds of taxes here: household and corporate income taxes, capital gains taxes, sales taxes, property taxes, value-added taxes, customs duties, any fees charged as a proportion of costs associated with providing services, etc. These do not include social contributions.
Public services. Some goods and services provided by the public sector can also generate revenue. This can include licensing bandwidth to telecommunications companies, revenues from state-owned enterprises like national mining companies in commodity-rich countries or national banks, utilities, etc. These are not always profit maximizing services and there tend to be laws or designations around where any profits from these services go.
Grants and contributions. Some governments receive grants from other governments or international organizations. This can include a government of a developing country, or a local or regional government that receives transfers from the national government. Social contributions can include voluntary or mandatory social security contributions, which by some definitions differ from taxes in that they are entitlements payed to the beneficiaries under specific events (old age, sickness).
Interest and property income. The government owns assets such as land that can often generate income through rent, for example. Some governments can act as creditors and lend money directly through public or quasi-public agencies that charge interest (usually lower than market rates). Central Banks can also buy and sell some assets and send any profit to the Treasury/Finance Ministry.
Other revenues. One time sources of revenue can include fines and penalties, sale of public assets, or lotteries.
