Text in equations in LaTeX

v = \frac{s}{t} \quad \text{for uniform motion}

\quad adds a space as wide as the letter M to separate the text from the formula. Inside \text spaces work normally; in math mode itself they are ignored.

T_{\text{boil}} = 373\,\mathrm{K}

A word subscript goes through \text, and a unit through \mathrm with a thin space \, before it. Both should be upright, since neither is a variable.

P = \frac{W}{t} \quad \text{where } W \text{ is the work done}

A variable inside the sentence goes outside \text: close the braces, type W and open \text again, remembering the spaces at the edges.

\mathrm{e}^{\mathrm{i}\pi} + 1 = 0

\mathrm is for single symbols that are not variables: the number e, the imaginary unit i or the differential d. Spaces inside \mathrm are ignored, so it is no good for words.

\operatorname{sgn}(x) = \frac{x}{|x|}

A function name LaTeX does not know, such as sgn or erf, goes through \operatorname from amsmath. It gets an upright font and the right spacing before the argument, just like \sin.

\begin{align}
  x &= 2 \\
  \intertext{and substituting into the second equation}
  y &= 4
\end{align}

\intertext from amsmath puts a sentence between the lines of an aligned block without breaking the alignment of the equals signs. The text starts at the left margin like a normal paragraph.