Net Force
Net force is the single overall force found by combining all forces acting on an object, accounting for their directions.
Definition
Net force is the total force on an object after you add up all the pushes and pulls: forces in the same direction add together, and forces in opposite directions cancel part of each other. It is the vector sum of all forces acting on the object, and it is the net force, not any single force, that determines acceleration through $F_{\text{net}} = ma$. Written out, $\vec{F}_{\text{net}} = \sum_i \vec{F}_i$, computed by adding force vectors component by component, and its direction is always the direction of acceleration. Free-body diagrams isolate one object and show every force on it. Resolving those forces into perpendicular components lets you write one equation per axis.
Example
Two friends push a box to the right with $10\ \text{N}$ each while friction pulls it left with $5\ \text{N}$, so the net force is $10 + 10 - 5 = 15\ \text{N}$ to the right. A $5\ \text{kg}$ sled pulled forward with $30\ \text{N}$ against $10\ \text{N}$ of friction has a net force of $30 - 10 = 20\ \text{N}$ forward and accelerates at $20 \div 5 = 4\ \text{m/s}^2$. With an angled pull, a $10\ \text{kg}$ crate pulled with $50\ \text{N}$ at $37^\circ$ above horizontal on a floor with $\mu_k = 0.20$ gets a horizontal component of $50\cos 37^\circ = 40\ \text{N}$; vertically, the normal force is $N = 98 - 50\sin 37^\circ = 98 - 30 = 68\ \text{N}$, so friction is $0.20 \times 68 = 13.6\ \text{N}$, the net force is $40 - 13.6 = 26.4\ \text{N}$, and $a = 2.64\ \text{m/s}^2$.
Key Insight
If the net force is zero, the object keeps doing whatever it was already doing, staying still or moving at a steady speed. A big force does not guarantee motion: a $500\ \text{N}$ push on a car is useless if friction and the brakes push back with $500\ \text{N}$, leaving zero net force and no acceleration. There is also a rule about which forces count. Internal forces between parts of a system always cancel in pairs by Newton's third law, so only external forces contribute to the net force on the system as a whole, which is why you cannot lift yourself by your own bootstraps.