Overview:  The returns of an investment are a measure of the rate of growth of the value of that financial instrument. In its simplest form, the return is the percent change in the market price of the instrument plus the percent gain from the interest or dividends received on the instrument. The divisor in determining the "percent change" is always the earlier point in time. Returns are always expressed as a percent and returns are usually measured over a one year period unless otherwise noted.

Historical  vs. Expected:  Returns may be measured either historically (i.e. in retrospect) or as an expected value (i.e. in the future). Historical returns may be measured precisely, as a matter of record, whereas expected returns may only be guessed at (albeit with elaborate predictive, and woefully inaccurate, models).

Cash:  Returns on a cash instrument is total amount of interest as a percent of the original investment. The market value of cash instruments tend to stay constant, but the interest that is earned may include "interest on the interest" such that the actual returns over time may be a greater rate than the quoted rate of interest. Historical models and expected models are often the same, because interest rates are often guaranteed, thus making calculations quite certain. See also: Rate to Yield Model.

Stocks: Returns on stocks (ke) is a similar concept. That is, the total historical return is the annualized percent gain (or loss) in share price plus the percent return from dividends. Two classic models used to estimate expected returns are the Capital Asset Pricing Model (CAPM) and Gordon's Model.

Portfolios:  Returns on a portfolio  (kport) may be calculated at the micro level.  That is, portfolio returns are equal to the change in the market value of all the securities in the portfolio plus all the dividends related to those securities, divided by the total beginning market value of the portfolio.  

As an alternative, the returns of the portfolio may be calculated at the macro level.   That is, the returns of the portfolio is the weighted average of the returns of the individual securities - weighted by the market value of the securities.  I call this the "kiwi" model because it looks like this:

 Wtd Avg =  Σ k_iw_i,   or the sum of the individual returns (k) times the individual weights (w)

Bonds:  There are three models for returns on bonds, Vb, WSJ , and Rodreguez's Model.