Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (Sallallahu Alaihi Wasallam) said,
"When a man dies, his deeds come to an end,
except for three:
A continuous charity,
knowledge by which people derive benefit,
pious child who prays for him.''
[Muslim].

Commentary: 
A thoughtful reflection of this Hadith will reveal that the three
things are indeed man's own good deeds which somehow continue to
exist even after his death. For example, places like a hospital,
a mosque, a religious school and a well built by a man continue
to operate like a trust as traces of his activity in earthly life.
To quote the Qur'an:

"And We record that which they send before (them), and their traces
[their footsteps and walking on the earth with their legs to the mosques
for the five compulsory congregational prayers, Jihad (holy fighting in
Allah's Cause) and all other good and evil they did,
and that which they leave behind].'' (36:12)

So long as they exist and people benefit from them, the dead person will keep
receiving his due reward. The same is pertinent to the spread of knowledge of
Islamic disciplines to people's moral uplift. For instance, if during his lifetime
he wrote books based on Qur'an and Hadith, advocated the cause of Islam, instructed
lots of pupils in Islamic teachings, it will be counted as an unending activity on
his part. To quote a Hadath: "The one who called people to adopt the Right Path will
receive reward in the same measure as did those who had the good fortune to follow him.
'' Besides, this Hadith tells us that, after his death, man will receive reward by the
prayers of his virtuous children, and other Ahadith too confirm it. This clears the issue
of sending the fruit of good works to the welfare of a dead person's soul. Charity or the
continual charity from the part of a dead person himself, beneficial (religious) knowledge
and prayer are acts of Sunnah under this rule. With the exception of these, all other deeds do not benefit the dead. However, if the dead person was under obligation to perform Hajj
or had missed obligatory or vowed fasts, his heirs are commanded by the Prophet (PBUH) to observe them on his behalf. Because this is like a debt payable by the dead person and his heirs are under obligation to discharge it for him.
Yet, this rule is inapplicable to other forms of physical worship.