There's an old joke along the lines of, "what's the difference between a sermon and a homily?" the response/punch line of which is "about ten minutes".
Sermon, homily, reflection, lesson, talk, address... these names and no doubt others are used to describe what is delivered in the act of preaching. Sometimes the name used reflects a specific tradition - RC and high Anglican churches tend to have 'homilies', protestant non-Confirmists tend to have 'sermons'. Sometimes it is used to get around various bits of ecclesial nonsense, such as the Baptist church where I was invited to "speak" at the service for the 'Ladies' Weekend' but not to 'preach' because I was a woman and therefore not permitted (by that congregation at that time) to do so. Sometimes it is used as a way of avoiding criticism that this isn't quite 'proper' - a reflection may not carry the (perceived) 'authority' of a sermon, a 'talk' may be acceptable from someone whose credentials we are less certain about... and so on and so forth.
Homiletics is the study of the art of preaching, homiletics is pretty much what happens when someone sits down to prepare whatever it is they are going to say, irrespective of what it is called or how long it will last, never mind how many people will hear it. So, it could reasonably be argued that whatever is delivered is, in fact, a homily, what differs is the style or duration.
A sermon is probably understood pretty much as 'proclamation' - or to use the Greek, kerygma. We know even as we listen that this is more than a 'lesson' or a 'talk' though it may well, and often needs to, include an element of teaching - or to use the Greek didache (from which we get our word didactic). Whilst these titles, and I have on occassion used the word 'talk', have worth and are far more accessible to non-church folk than 'sermon' or 'homily', there is a danger that some will perceive them as somehow lesser, that the mystical element is missing. At worst, perhaps it is, but we need to be careful, I think, not to confuse style and purpose with content: sometimes rather than a sermon what is needed might be a talk or a lesson, information to be acquired. Visits from Misison Partners, videos used in speical services - these may legitimately displace the 'sermon' in favour of information sharing.
Reflection is a word that is often used when what is shared is not explicitly expository, that is does not seek to explain and expound the text, but instead springs from the preacher's engagement with it. Story-telling (narrative) sermons are sometimes named reflections. I use the term sometimes when I feel that the purists who value exposition might feel short-changed by what I'm offering because it is less 'scientific' and more 'organic' (finding words to descirbe it is tricky).
Each of the terms has value, and each can refer to a specific style or purpose of delivery, but always what is offered is a response to scripture and experience, to an exploration by one person who has some purpose (a theme, a series, a key word or phrase...) and seeks to share their discoveries, thoguhts or insights with others. We trust, whether it is a story, a three alliterative point exposition, a response to world events in the light of a text, whether itis pastoral or prophetic, encouraging or challenging, that somewhere in the process God is active.
At one level, I don't think the names matter one jot. At another, perhaps they do: if they are used, consciously or otherwise, to categorise sermons as 'proper' or not, as 'good' or not, as 'this duration' or not, as of value or not, then we have a problem we need to overcome. I'm not sure what name might be the most helpful, be accessible to people who don't know church, capture something of the mystery... but whatever name we use, as preachers and preachees we need to value it.
Of course, a further question remains - must or should there be preaching at all... but that'll have to wait for another day!