What is the relation between Variance and Co-Variance? Explain with example

Variance measures the variability (volatility) from an average or mean of a data set, it can be a measure of risk of investment return.

Covariance measures of the degree to which returns on two risky assets move together.

- to reduce the risk in an investment portfolio choose assets with negative covariance -

1. Variance is always non-negative: a small variance indicates that the data points tend to be very close to the mean (expected value) and hence to each other, while a high variance indicates that the data points are very spread out around the mean and from each other.

Covariance is a measure of how changes in one variable are associated with changes in a second variable. Specifically, covariance measures the degree to which two variables are linearly associated. However, it is also often used informally as a general measure of how monotonically related two variables

A covariance refers to the measure of how two random variables will change together and is used to calculate the correlation between variables.

The variance refers to the spread of the data set — how far apart the numbers are in relation to the mean, for instance. Variance is particularly useful when calculating the probability of future events or performance. 

Simply put,

Variance is for only1 population or share and it is about measuring of dispersion of its probabiility away from the mean (how much the results differed from the mean).

Covariance (adding "Co") is for2 populations or shares and it is about measuring the relationship of the dispersion for both the population (how much the results differed for both together).

Covariance is more about portofolios as portofolio managers tend to seek the least risk, hence focusing on the relationship between the volatility of items within the portofolio.

we can simple the answer between the relation between risk and return ( the variance between expected rate )in one share and portofolioso the covariance is to measure the volatility of return together with the return of securities