Most of what you see being posted is a verbal commitment which are legally non-binding. However, since the player and the school usually have done their research the prospects are very good that the player will in fact attend that school. Contingencies such as SAT scores and maintaining a certain GPA often apply.
The binding commitment is the National Letter of Intent which is a legal contract between the player and the school for D1 and D2 players. Since there are no athletic scholarships in D3 an NLI is not used.
There is certainly an element of honoring your word involved and for the most part it works. Trouble starts when coaches get fired or leave. All the more reason to ask poignant questions about how your child fits into the school's program.