Rad Zdero, in his Nexus: The World House Church Movement Reader does about as good a job as anyone in answering this question.
Many believers today participate in 'small groups', such as Bible study groups, prayer groups, accountability groups, affinity groups, etc. However, 'small groups' are often utilized differently in various types of...traditional churches, cell churches, and house church networks. Small groups in all three styles of churches usually meet in homes and encourage the participation of believers. But, that's where the similarities end. While we must clearly recognize and celebrate the hand of God in all manner of churches, there are important differences between traditional churches, cell churches, and house churches that should be understood.
On one end of the spectrum, for instance, is the traditional church...[that] uses small groups (often misnamed 'cell groups')--this can be described as a 'church WITH small groups.'
Further along the spectrum is the cell church that places an equal or greater emphasis on its mission-minded small groups (properly called 'cell groups') compared to its weekly large group services--this can be described as a 'church OF small groups.'
However, the house church network sees each house church as a fully fledged, autonomous, church in itself--'church IS small groups'.