The big difference is that professional grade polyurethane foam is made from two components (polyol and isocyanate) that chemically react to make polyurethane foam and sold in twin tanks. Foams sold in DIY stores are single component (one can) that rely on air moisture to cure.

Single component foams are ‘open cell’ in structure. Open cell foams  afford no water resistance and little in the way of insulation since the cells allow absorption (like a sponge) and transmission of water and allow air circulation thus heat energy is lost by convection. Single component foams are typically not fire rated and will be highly inflammable.  Apart from perhaps void filling in dry areas, single component foams should not be used in applications where water resistance and thermal insulation is required. Single component foams do not adhere to substrates as well and in the small cans sold in DIY stores offers a very poor cost per metre when compared to two component foams.