yes incomplete combustion is a result of a lack of oxygen. the possible products of incomplete combustion are usually Carbon Monoxide (CO(g)) and Carbon Soot (C(s)).
Basically what the other people said - not enough oxygen. Example of this is: Complete C<SUB>5</SUB>H<SUB>12</SUB> + 8O<SUB>2</SUB> * 5CO<SUB>2</SUB> + 6H<SUB>2</SUB>O Incomplete C<SUB>5</SUB>H<SUB>12</SUB> + 5½O<SUB>2</SUB> * 5CO + 6H<SUB>2</SUB>O C<SUB>6</SUB>H<SUB>12</SUB> + 3O<SUB>2</SUB> * 5C + 6H<SUB>2</SUB>O C<SUB>6</SUB>H<SUB>12</SUB> + 4½O<SUB>2</SUB> * CO<SUB>2</SUB> + CO + 2C + 6H<SUB>2</SUB>O