Examples:
3xy and -5xy are like terms (same variables).
3xy and -5xym are not like terms (the second term has a variable that the first doesn't).
-2m2xh and 4m2hx are like terms (same variables and exponents, just in different order).
-2m3xh and 4m2xh are not like terms (the variable m has different exponents in each term).
If there's more than one term separated by plus or minus signs, then we have an expression. For example:
5x2y – 3xy + y + 5
This expression has four terms: 5x2y, -3xy, y (or 1y), and 5.