Mortality rate is a measure of the number of deaths (in general, or due to a specific cause) in a population, per unit time. Mortality rate is typically expressed in units of deaths per 1000 (or 100'000) individuals per year; thus, a mortality rate of 9.5 in a population of 100'000 would mean 950 deaths per year in that entire population, or 0.95% out of the total.
The crude death rate reflects the age- (and gender-) specific mortality rates and the age (and gender) distribution of the population.
In contrast, the often used standardised mortality rate (SMR) represents a proportional comparison to the numbers of deaths that would have been expected if the population had been of a standard composition in terms of age, gender, etc. In other words, this would be the mortality rate in your population if it had the same age and gender distribution as a (defined) standard population.
Everitt, B.S. The Cambridge Dictionary of Statistics, CUP. ISBN 0-521-81099-X