Below are some examples to explain services usage by using HTTP/GET and KVP encoding.

Basic WMS requests
Request / Description Result
GetCapabilities

Retrieve capabilities information

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WMS 1.1.1 GetCapabilities

${default.online.resource}?request=GetCapabilities&version=1.1.1&service=WMS
GetMap

Retrieve a map from a WMS

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WMS 1.1.1 GetMap

A simple GetMap Request: layers = StateOverview
styles = default style

HTTP KVP Request:

...services?
REQUEST=GetMap
SERVICE=WMS
VERSION=1.1.1
WIDTH=250
HEIGHT=250
LAYERS=StateOverview
TRANSPARENT=TRUE
FORMAT=image/gif
BBOX=210625.44883700498,4175983.9939139457, 697811.8183418702,4663170.363418811
SRS=EPSG:26912
STYLES=
WMS 1.1.1 specialized GetMap

A specialized GetMap request

layers = StateOverview, Vegetation, Lakes, Roads
styles = default and BasicRoadsStyle

HTTP KVP Request:

...services?
REQUEST=GetMap
SERVICE=WMS
VERSION=1.1.1
WIDTH=300
HEIGHT=238
LAYERS=Vegetation,Lake,Roads
TRANSPARENT=TRUE
FORMAT=image/png
BBOX=372233.9342431239,4460438,489069,4552689
SRS=EPSG:26912
STYLES=default,default,default
GetFeatureInfo

Some examples for GetFeatureInfo requests
(simulating a "click" on the map of a GetMap request):

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WMS 1.1.1 GetFeatureInfo StateBoundary + Counties

A GetFeatureInfo request with html output, (Layers 'StateBoundary' and 'Counties')

HTTP KVP request:

...services?
REQUEST=GetFeatureInfo
VERSION=1.1.1
X=236
Y=201
WIDTH=404
HEIGHT=369
QUERY_LAYERS=StateBoundary,Counties
LAYERS=StateBoundary,Counties
FORMAT=image/jpeg
INFO_FORMAT=text/html
BBOX=449822.375,4347902.5,457250.9375,4355331.0
SRS=EPSG:26912

 
WMS 1.1.1 GetFeatureInfo Count Boundaries + StateBoundary

A GetFeatureInfo request with GML output, (Layers 'StateBoundary' and 'Counties')

HTTP KVP request:

...services?
REQUEST=GetFeatureInfo
VERSION=1.1.1
X=199
Y=123
WIDTH=469
HEIGHT=354
QUERY_LAYERS=StateBoundary,Counties
LAYERS=StateBoundary,Counties
FORMAT=image/jpeg
INFO_FORMAT=application/vnd.ogc.gml
BBOX=273167.28125,4594791.5,285628.8125,4607253.0
SRS=EPSG:26912

 
DescribeLayer

Some examples for DescribeLayer requests:

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WMS 1.1.1 DescribeLayer StateBoundary

A DescribeLayer request on Layer 'StateBoundary'

HTTP KVP request:

...services?
service=WMS
version=1.1.1
request=DescribeLayer
Layers=StateBoundary

 
WMS 1.1.1 DescribeLayer Roads

A DescribeLayer request on Layer 'Roads'

HTTP KVP request:

...services?
service=WMS
version=1.1.1
request=DescribeLayer
layers=Roads

 
GetLegendGraphic

Get a legend graphic for the chosen layer

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WMS 1.1.1 GetLegendGraphic Roads

A GetLegendGraphic request on Layer 'Roads'

HTTP KVP request:

...services?
service=WMS
version=1.1.1
request=GetLegendGraphic
layer=Roads
format=image/jpg