Gross private domestic investment is the measure of investment used to compute GDP. This is an important component of GDP because it provides an indicator of the future productive capacity of the economy. It includes replacement purchases plus net additions to capital assets plus investments in inventories. It usually amounts to between 15 and 18 percent of GDP. Net investment

Net investment

In economics, net investment refers to an activity of spending which increases the availability of fixed capital goods or means of production. It is the total spending on new fixed investment minus replacement investment, which simply replaces depreciated capital goods....

 is gross investment minus depreciation.

Gross private domestic investment includes 3 types of investment:

Non residential investment: Expenditures by firms for machines, tools and so on...

Residential Investment: Includes expenditures by households and firms on apartments, buildings, new factories...

Change in inventories: The change of firm inventories in a given period. (Inventory

Inventory

Inventory means a list compiled for some formal purpose, such as the details of an estate going to probate, or the contents of a house let furnished. This remains the prime meaning in British English...

: is the goods that are produced by firms but kept to be sold later ("stock" in British English.)