Definitions first:
1. A will is a testament. 
2. A trust is a legal relationship established between the person creating it (settlor), the person or people receiving benefits from it (beneficiaries) and the person administering it (trustee). 
3. A testamentary trust (will trust) is a trust created by the execution of a will.

There are other types of trust but a testamentary trust has a number of unique aspects because it arises from a singular and undesirable event: the death of the settlor.  If you had to die to create it, the legal and tax systems are much more willing to treat such a trust as being legitimate and potentially advantageous.  For example, in your will you can specify that your sister be the trustee of a trust for the benefit of your son and that your son's education and well being be paid for, but that he not receive a lump sum of money until he has completed a university degree and worked for one full year, paying his own cost of living.  In another example you could create a trust which would let your second spouse enjoy the use of your vacation property for life, but that your child would inherit the property after the spouse's passing.  In short, you can create a trust for almost any purpose imaginable.

Very few lawyers write wills and create trusts with a complete view of income tax in mind.  Most wills are written is boilerplate format with a view to the expedient distribution of the estate and the minimization of probate taxes (up to 1.5% of the estate value).  People name beneficiaries on life insurance policies and RRSP's, hold assets in joint ownership, make gifts in anticipation of death and are otherwise maniacally focused on minimizing the costs of transferring assets to their chosen beneficiaries.  These taxpayers and their lawyers almost never think about the income tax implications of their decisions since tax will be on the beneficiaries not the estate!

Let's say you had an estate worth $1,000,000 composed of a house, cottage, investments, life insurance, etc. and a spouse and three minor children.  The normal plan sees all assets transferred directly to the spouse with minimal costs.  The spouse pays debts and invests the balance (say $750,000) to produce a long term income and the spouse has a modest work income of $32,000.  Each dollar of investment income will be taxed in the spouse's name at a marginal tax rate of 31% or higher, depending on clawbacks of child tax benefit and medical expenses.  In other words, a $30,000 investment income would incur income taxes of about $12,000 per year.  OUCH...

If the will had instead specified the creation of several trusts, one for the spouse, one for the family, one for each child and one from life insurance, then the investment income would all be in a 21% marginal tax rate and a total of about $23,000 per year could be earned by the children's trusts with absolutely no income tax.  The family tax burden could be reduced by about $10,000 per year for many years to come and the spouse would save taxes for life.  That's $10,000 per year less for the tax collector and $10,000 per year more for your spouse and children.  Sounds good?  Just contact the office if you'd like to learn more about the many income tax and estate control advantages of will trusts.