Addition is combining two or more numbers to find their total. The result is called the sum.
Column addition: line up digits by place value and add from right to left, carrying any tens into the next column.
$$\begin{array}{r} 347 \\\\ +\ 286 \\\\ \hline 633 \end{array}$$
Adding decimals: keep the decimal point aligned.
$$4.7 + 3.85 = 4.70 + 3.85 = 8.55$$
Adding negative numbers: $$5 + (-3) = 5 - 3 = 2$$ $$-4 + (-2) = -6$$
Adding fractions: find a common denominator first. $$\frac{1}{3} + \frac{1}{4} = \frac{4}{12} + \frac{3}{12} = \frac{7}{12}$$
Common error: not aligning place values in column addition, or forgetting to carry.