A program is defined as a group of related projects, subprograms, and program activities managed in a
coordinated way to obtain benefits not available from managing them individually. (PMBOK 5th, 2013, P. 9)

A portfolio refers to projects, programs, subportfolios, and operations managed as a group to achieve strategic objectives. The projects or programmes of the portfolio may not necessarily be interdependent or directly related. (PMBOK 5th, 2013, P. 9)

"a temporary endeavour undertaken to create a unique product, service or result." (PMBOK 5th, 2013)

e.g. The required group of projects to deliver the Olympic games, comprising of a group of different related projects such as stadium, facilities, railway etc..

e.g. an isolated project like building a bridge

e.g. the group of projects required to develop an oil refinery

e.g. the overall portfolio of an oil and gas company of upstream and downstream facilities

Also:

An integrated, structured-framework that coordinates, aligns and allocates resources, and plans, executes and manages a number of related projects to achieve optimum benefits that cannot be realized if the projects are managed separately.
(Shehu & Akintoye, 2009)

companies might choose to organise their projects into portfolios to maximise their internal and external resources.

A portfolio approach is also used to distribute risks

A portfolio structure allows companies to allocate resources more efficiently

Portfolios allow companies to develop an inter-project perspective.