There are lots of elements associated with a venture investment.  However, the most prominent consideration is valuation.  At the highest level there are three components in this calculation: pre-money valuation, new equity, and post-money valuation.

Pre-Money Valuation
The pre-money valuation refers to the value of the company before taking new money (the investment) from the VC.  At its essence this metric reflects the stand-alone value of the company at the time of investment. 

New Equity
New equity is all of the capital that converts to equity during the investment round.  This includes new capital from investors (VCs, angels or otherwise) and all debt that converts to equity in this round. 

Figuring out the appropriate amount of new capital to raise is complicated, as both the entrepreneurs and investors have conflicting incentives; each are incented to both raise more and less.  Finding the right amount usually results from a thoughtful and well-intentioned dialog between the two parties.

Post-Money Valuation
The post-money valuation is the value of the company after the investment has been made.  This value is equal to the sum of the pre-money valuation and the amount of new equity.  The intuition behind this is pretty simple.  The company didn't lose any value during the transaction, so the stand-alone (pre-money) value of the company remains the same.  And a dollar is worth a dollar whether it's in the pocket of a VC or in the coffers of a startup.  As a result, the value of the company post-investment (post-money) is the pre-money plus new equity.