Risers are the vertical elements that support the leading edge of the upper tread and are set on the lower tread.
Treads are the horizontal elements we step onto. The rear of the step fits under the upper riser and the leading edge is rounded and hangs over the lower riser and is completed with a nosing.
Newel post is the end piece of the vertical element at the end of the handrail.
Balusters or spindles are the bars supporting the handrail.
Reinforcing Blocks and Wedges are fitted under the stairs to keep the risers and treads in place.
To remedy to creaks in the stairs, which are generally caused by humidity and wear, one can usually mend the wooden elements that have moved from under the staircase. Make sure that there are no loose wedges between the end of the treads and risers, if so you may glue them or screw them back into place.
If there is no access to the bottom of the stair you will have to repair the stair from above. You may screw back the leading edge of the tread if it has come away from the riser.
If you need to change balusters you may do so by taking the bottom spacer away from the baluster behind the one you intend to remove, change it then re-insert it.