Children require speech and language therapy for a variety of reasons. Some children have difficulty making sounds in words which can make them difficult to understand, and need help to find the correct placement for tongue, lips and teeth.  Some children have a difficult time with language, either in understanding or in expressing themselves.  Some children have difficulty with "stuttering", also known as "fluency", and some have memory or word finding problems.  A wide variety of physical or organic problems require speech therapy as well, including mental retardation, apraxia or dysarthria. Some children have difficulty recognizing or understanding social cues needed for smooth and effective social interactions, known as "pragmatics". Other children have the challenges of autism. And, of course, children can and often do present with a mixture of challenges.  Without assistance these children risk falling behind academically as well as socially, and may be shy to participate in classroom activities, or perhaps not participate at all!
With help, children can learn skills, strategies and techniques that can give them the assistance they need to rise above whatever challenges they face.  Some children may need direct therapy, anywhere 1-3 times a week, for 15-30 minutes, while others may be better served by a consultative model of therapy, where I consult with the classroom teacher on ways to support a child in the classroom. There is always collaboration with the child's teacher to work to carry over success in therapy into the classroom, working toward generalization of learned skills throughout the school (and home!) environment.  
Articulation:  Articulation is the production of speech sounds, and "intelligibilty" simply means how well a child is able to be understood.  Children can make individual errors in making sounds, or they can make sound errors in patterns known as "disordered phonological processes".  A common one is Liquid Simplification, a process where the "rule" the child uses is all /r,  l/ sounds are made as /w/; the Elmer Fudd Syndrome; "you Wascally Wabbit!"
Language: Expressive language looks at how well a child can express him or herself.  This includes vocabulary skills, including concepts and word finding, as well as grammar usage in connected speech.
Receptive language looks at how well as child is able to understand what is being said and includes concepts and following directions,  as well as recognizing grammatical structures in use.
Pragmatics: Pragmatics is the use and recognition of the "non-verbal" aspects of communication, such as body language, facial expressions, situational cues, social register (you talk to your friends differently than your principal),  and figurative language or idioms. Pragmatic language also covers conversational skills such as topic introduction, maintenance and how to leave a conversation , as well as appropriate topics of conversation