Of course there is no single, authorized definition of "simple church." Some use the adjectives "simple", "organic" and "house" interchangeably, though many of these churches do not actually meet in homes. (The last one I started first met under the covered entryway of a defunct Wax Museum and now meets in a backyard.)

The most important characteristic that distinguishes simple churches from more traditional ones can be illustrated like this (click on the link):
Two Drawings

The essential difference between these two drawings is how the Lord gives his people what they need in order to meaningfully connect with him and help each grow. In the first he is expected to give most (or all) of it to them through a primary leader. In the second he is expected to give it to them through each other. It's really as simple as that. As the scriptures say:

"From Jesus the whole body, joined and held together by every supporting ligament, grows and builds itself up in love, as each part does its work" (Ephesians 4:16).

"There are different kinds of gifts, but the same Spirit. ...To each one of us is given the manifestation of the Spirit for the common good" (1 Corinthians 12:4, 7; cf. 1 Peter 4:10-11).

In other words, the invisible Spirit of God who lives in every believer reveals ("manifests") himself by moving us to respond to each other's needs and give gifts to each other, gifts of a spiritual nature which both express his love and build us up. Every time we're together is supposed to be like Christmas!

Another good way to understand the simple church model is to contrast it in more detail with the standard, conventional church model. Please understand, the terms “conventional” (“traditional/institutional”) and “simple” (“organic/ house”), by themselves, are not meant to convey a positive or negative impression. They are just ways of referring to substantially different kinds of churches, values and ways of doing things. Click on this link for a chart contrasting:
Conventional and Simple Churches