Ground is considered the reference point of a circuit. Ground is often referred to as negative, but in some circuits there can be negative voltages below ground voltage, sometimes by several hundred volts as found in tube electronics. In general, ground is considered zero volts, but only in reference to the circuit in which it's being measured.
In digital electronics, ground is usually the negative terminal of a power supply (whether it's a bench supply, battery, solar cell, etc.). In these circuits, ground is negative in the sense that current is flowing back to the power supply (in conventional current, anyway. See below.). It's important to note the difference between digital ground (such as a battery operated device, where the ground is only in reference to the circuit the battery is powering, or "floating ground") and earth ground, which is simply an electrical reference to the earth itself (such as the wiring inside your home, where ground and negative are not interchangeable).
One thing to keep in mind when designing a circuit is whether components are documented for conventional current or electron flow. When Benjamin Franklin was writing up the conventions for electricity, he stated that charge flows from positive to negative, and it remains convention to this day. As it turns out, charge actually flows from negative to positive (the reasons for which are not under the scope of this article), and while most components and data sheets are rated for conventional current, it's important to understand the distinction as some technical books use electron flow to represent concepts with more clarity, and in fact, many electronics textbooks have a version for each convention.
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