Fig 1. Layout of ground connection for various instruments. (1) shows ground connected to scope. (2) shows ground connections of power supply. (3) shows the BNC to binding post converter connected to the function generator.

It is also strongly recommended that you read the tutorial  Grounding and Grounding  Practices to become familiar with terminology and to clarify the concept of ground.

Voltage measuring instruments measure only differences in potential between two points. When making voltage measurements some point must be established as a reference so that voltages measured at various points in a circuit can be related to each other. If an "earth" ground used as a reference, it is accepted to be at zero potential or zero volts. Thus, a voltage measurement will read the same at any distant  site using the ground as the reference. Generally, when one specifies a voltage it is understood to mean with respect to ground , unless otherwise noted.

In usage "ground"  has come to mean a physical connection point which is common to all circuits and to any instrumentation connected to the circuits; hence,  proper terminology refers to the common point and not "ground" unless an earth ground is provided.  The terms "ground" and "common"  are frequently used interchangeably.

In the following figure the ground connections of the power supply unit are shown in detail. 

Fig 2. Ground connections for power supply

As you can see, it is important to make all votage measurements with respect to one common ground. This is shown in (2) of Fig 1.

When using BNC cables the common point is provided by the outer metallic  casing of the cables. These may or may not be connected to true "earth" ground.

Fig 3. A BNC jack and co-axial cable

(If you read the grounding tutorial this should cause no confusion.) Connecting a function generator to an oscilloscope using a BNC cable establishes the common point for both instruments. Any voltages displayed on the scope will be  referenced to this point. If measurements are now made with a multimeter, the low side of the meter is connected to this point  as the voltage reference point and all voltages are measured with repect to this point. In the case of a power supply, the reference terminal can be either the negative or positive terminal, depending on what voltage the circuit requires.

The oscilloscope is also connected to a circuit using a 10:1 probe which increases the input impedance to 10 Megohms and provides compensation to view high frequency transient waveforms.  The probe end is equipped with a ground lead having a miniature alligator clip.   In high frequency circuits this lead should be connected to ground on the circuit board close to the point of measurement. At the lower frequencies normally used in this lab the ground clip could be lef disconnected provided a banana lead cable connects the scope to the supply ground. (Look at "1" block in the above diagram. In addition to providing a ground to the scope, it also reduces cable clutter. When using this ground approach, the probe ground lead can be  attached to the cable using the alligator clip to get it out of the way.