Harassment

Harassment is the process of damaging enemy Heroes during the laning phase to keep them from farming, to gain lane dominance or to drop their HP to prepare for a kill.
Strong harassment can force the enemy to play extremely defensive, spend a lot of money on consumables, repeatedly return to their base, and miss out on experience and last hits.

Means of Harassment

The most basic form of harassment is simply damaging the enemy Hero with physical attacks. This is most easily accomplished if your hero is a ranged one (with a greater range allowing for a greater degree of harassment) while the enemy Hero or Heroes are melee or have low range. Directly attacking an enemy Hero will draw creep aggro, which means that nearby enemy creeps will attack you. This can be partly prevented by right-clicking the ground to move into attack range, performing an attack and immediately retreating to avoid being damaged by creeps.
When manually used, castable attack modifiers like Viper's Poison Attack or Huskar's Burning Spears will not draw creep aggro at all, making them extremely powerful harassment tools.
Most damaging spells can be used for harassment purposes, and are especially effective because they do not draw creep aggro. Especially suited are slowing abilities like Crystal Maiden's Crystal Nova because they allow the user to perform some additional physical attacks on a retreating enemy, as well as spammable abilities with a low mana cost such as Clockwerk's Rocket Flare. Experienced players usually integrate this technique into their farming: A well-placed AoE Ability can get a lasthit on a creep while simultaneously damaging an opponent.

Items that provide some early mana regeneration (e.g. Soul Ring, Clarity Potions or Arcane Boots) can greatly increase the harassment potential of a Hero. Although ranged Heroes are generally better suited for physical harassment, an Orb of Venom can allow some melee Heroes like Riki to harass effectively.

Countering Harassment

Physical harassment should be countered with items that provide Damage Block, HP regeneration or Armor: A Poor Man's Shield, combined with a Ring of Health can render a Hero almost immune to regular harassment.
Harassment with Spells can be mitigated with a Magic Wand, as they will constantly gain charges and provide the holder with a steady supply of Mana and HP.
Some abilities are excellent counter-harassment tools. If a player faces heavy harassment, he might consider adapting to a defensive skillbuild, e.g. investing as many skillpoints as possible into Dragon Blood or Backtrack.
In some laning situations, it is better to force a small skirmish and attempt to kill an enemy Hero instead of just passively allowing the enemy to gain lane dominance through constant harassment. For example, a Vengeful Spirit and Pudge lane will quickly lose the lane against Drow Ranger and Ancient Apparition in terms of harassment. However, they have a distinct advantage if they try to kill their opponents instead of just passively laning against them.

Stout Shield (250)	Tango (125)	Ring of Regen (350)	Magic Stick (200)
Poor Man's Shield (550)	Healing Salve (115)	Ring of Health (875)	Magic Wand (509)
Ring of Protection (175)	Bottle (650)	Helm of Iron Will (950)	Tranquil Boots (975)