Broadly, as I understand it, American schools are divided into primary/junior school, middle school and upper schools. Most British schools are just split into primary and secondary. Generally, children move from primary to secondary at about age 11. Through schools in HK offer the entire programme from entry at say reception to when the 'children' are ready for 3rd level education (e.g. CIS, GSIS, HKIS etc). Kellett is one of the schools that only offers Primary education at the moment.
Generally, reception is for 4 year olds, pre-reception would be for 3 year olds. It becomes confusing because different schools follow different education systems and have different cut off dates, for example in some schools, the child needs to be 4 before August/September to enter reception, at others, they operate on a calendar year. Pre-reception is also known as Kindergarten 1 (e.g. GSIS) and Reception is called Kindergarten 2.
Some schools, e.g. CDNIS, have an additional year between reception and Year/Grade 1 - called Prep. Just to add to the confusion, there are schools that even operate on a different school year altogether (e.g. Australian International School uses the Australian school calendar, so longest holidays are taken around Christmas).
Clear as mud? The only real way of getting a clear picture is to arrange a few school visits and examine the detail of the schools that appeal to you most for your child.
Because your child is so young, you should be in better shape. As I understand it, most of the waiting list problems are with entry further up the school. Generally, the schools are selective, so once your child is registered on time and passes the assessment, it is easier to get a place at the first possible year of entry (at least that is our experience). Kellett is one of the exceptions to this rule, as it is non-selective and entry is driven by a combination of when your child's name is put on the waiting list and the type of debenture held. As you go beyond reception, places are only available as people leave the school (e.g. CIS only interviewed debenture holders and siblings for Year 1 places this year - 20 people for 2 places, as I understand it). Good luck.