The IRS assumes that LLCs with more than one member are partnerships for tax purposes. That means the LLC itself pays no tax, but taxable profits and deductible losses are passed through to the members,who are treated as partners under the tax rules.
So at tax time to keep the IRS happy an LLC files Form 1065: Partnership Return of Income. The annual Form 1065 must also include a Schedule K-1 for each member. Schedule K-1 reports the member’s share of LLC income, deduction, and tax credit items. These amounts are then included on the member’s personal tax return.
Here's a detailed article that discusses LLC taxes tax treatment of LLCs