Negative for long options - the rent you pay for owning gamma and vega.
Accelerates near expiry for at-the-money options, because most of their remaining value is time value and it must reach zero at expiry. Out-of-the-money options decay more steadily and are near-worthless well before expiry, so the famous decay curve is an at-the-money phenomenon rather than a universal one.
The framing to use: theta and gamma are two sides of one trade. You are paid theta for being short movement, or you pay it for being long movement.