It may seem funny but the main difference between the two is the letter C. C stands for Covariance and this already makes a lot of changes with how these two can be used. When you say ANOVA, this is a type of technique that you can use when you are researching different topics that would need samples.
ANOVA can be used when you are trying to figure out the difference between two different types of populations. You will be using ANCOVA when you would like to compare a specific variable in two different populations.
You may also need to know the other variables that are related to this. The more that you know, the better. In ANOVA, the covariate is ignored but in ANCOVA, the covariate is considered.