Units are the references we use to measure real world phenomena. They are either defined in relation to real world measurement, or in relation to other units.
Quantity
Units themselves do not carry quantities, nor do quantities carry units. That said, both must be notated for concrete measurements. Twelve is a an abstract quantity, and egg is an abstraction of a physical item. A dozen eggs, on the other hand, is something you can buy at the grocery store.
Quantities are often rounded to an extent. Three decimal points or three digits of precision is usually a good rule of thumb where precision is not required. Excessive precision makes you look like a knowitall, or a conspiracy theorist.
Examples of notation for the frequency 20,327 Hz:
Notation  Written 

Plain  20,327 Hz 
Prefixed  20 kHz 
Scientific  2.0327×10^{4} Hz 
Engineering  20.033E3 Hz 
Prefixes
Quantities are often specified by common prefixes.
From Wikipedia.
Prefix  Symbol  Factor  Power 

tera  T  1000000000000  10^{12} 
giga  G  1000000000  10^{9} 
mega  M  1000000  10^{6} 
kilo  k  1000  10^{3} 
hecto  h  100  10^{2} 
deca  da  10  10^{1} 
(none)  (none)  1  10^{0} 
deci  d  0.1  10^{1} 
centi  c  0.01  10^{2} 
milli  m  0.001  10^{3} 
micro  μ  0.000001  10^{6} 
nano  n  0.000000001  10^{9} 
pico  p  0.000000000001  10^{12} 
Scientific notation
Scientific notation specifies quantity in the format of a × 10^{n} for 1 ≤ a < 10 and any integer n.
The *10^n
can be replaced by En
, e.g. 48,000
= 4.8*10^4
= 4.8E4
Engineering notation.
Engineering notation is the same as scientific notation, but with exponents
only divisible by 3. This makes them align with prefixes and makes verbal
communication easier. Reusing the above example, 48,000 would be written as
48*10^3
or 48E3
.
SI base units
Most base units have a historical definition grounded in simpler measurements. In modern times, they are put in relation to very specific physical constants in order to increase precision and to account for modern scientific theories.
For more, see Wikipedia: International System of Units.
Time
Time has the common symbol t
.
Unit  Symbol  Defined by 

second  s  Historically, a division of the day 
Length
Length has the common symbol l
.
Unit  Symbol  Defined by 

meter  m  Historically, 10,000 km was the distance from the equator to the north pole 
inch  in or “  2.54 cm 
foot  foot or ‘  12 in 
yard  yd  3 feet 
mile  mi  1,760 yd 
Swedish mile  10km 
Mass
Mass has the common symbol m
. It is commonly referred to as weight, but
weight is rather force caused by gravity.
Unit  Symbol  Defined by 

gram  g  Historically, 1 kg is the mass of one liter of water. 
pound  lb  circa 0.454 kg 
ounce  oz  1/16 pound 
stone  st  14 pounds 
Temperature
Temperature has the common symbol T
.
Unit  Symbol  Defined by 

Kelvin  K  The same scale as celsius, but 0K, absolute zero, equals 273.15°C 
Celsius  °C  Linear scale where 0°C is the freezing temperature of water and 100°C is the boiling temperature of water 
Farenheit  °F  Exact origins vague, but temperature defined in relation to Celsius as 5/9(x − 32) °C

Rankine  °Ra  As Kelvin is to Celsius, T_{°Ra} = 5/9 × T_{K} 
Current
Current uses the symbol I
.
Unit  Symbol  Defined by 

Ampere  A  Historically, coloumb per second 
The SI definition of ampere was formerly charge over time, with ampere being defined as coloumb per second. Later revisions has reversed the relation, and an ampere is technically defined directly as 6.241509074×10^{18} electrons per second.
Unnamed SI derived units
A few units relatively fundamental to the human experience don’t have SI names and are just referred to by their relation to the base units. However, historical named units may exist.
Velocity
meter per second knop
Acceleration
meter per second per second
Area
square meter hectare american football fields
Volume
cubic meter liter ounces
Named SI derived units
Force
Energy
Pressure
Rotation
Frequency
Electrical
Charge
Voltage
Voltage (sv: spänning, literally “tension”) measures electrical potential of charge.