As with most concepts in the transgendered/transsexual world, a definition of FTM must be dynamic to match the persons whom it describes. However:
The abbreviation FTM is more commonly heard than the term for which it stands, female-to-male, in a non-clinical environment. Specifically, and thus being somewhat of a misleading term, it refers to a transsexual person who has transitioned; that is, or moved from (a) Female (identity and form) To Male. More generally, it means a person who, identified female at birth or identified intersexed at birth, and raised as female, now identifies as, or identifies with aspects of, male, masculine and/or man. Generally, this person is transgendered, and in some narratives of transgenderism and transsexuality, a person who self-defines hirself as FTM (likewise by MTF) is, by definition, a transgendered person.
(See MTM/male-to-male for an alternative view.)
An FTM person will often use male pronouns or gender-neutral pronouns when referring to himself/hirself, and may choose a masculine or gender-neutral name. Likewise, but not exclusively, an FTM guy is generally masculine in appearance, and may undertake body modifications such as hormone therapy (testosterone), chest reconstruction, metaoidioplasty or phalloplasty to physically transition to male. However, such physical modifications are not necessary for a person to identify or be described as FTM, even though in countries where transsexuality is legally recognised, some level of physical transition is often necessary to gain legal recognition of one's sex (ie. to change the 'F' on your passport and birth certificate to an 'M'). In many cases, an FTM transgendered person will never physically transition (sometimes known as no-ho FTMs), or may undertake limited body modifications, such as low doses of hormones (sometimes known as lo-ho FTMs).
As above, FTM transsexuals tend to be more invisible in society than MTFs because they assimilate better post-transition. Testosterone, being the powerful drug that it is, produces profound and irreversible changes in the body, which will render a body that has artificially experienced its effects with an external appearance similar to a body that has gone through a natural male puberty.
On average, the only outward difference in appearance between the FTM male population and the 'factory-equipped' male population is that FTM guys tend to be shorter in stature, with smaller hands and feet. But generally, FTM guys tend to pass better in Western Society than their MTF sisters (who ironically find it more difficult to pass than FTM guys because of the effects of testosterone on their bodies!). Even without physical changes, genderqueer and no-ho FTMs - who are generally viewed by mainstream society to still be female - have, along with women in general, greater permission (than men identified male at birth) to be androgynous, and therefore pass more readily as male than MTF women do as female, in the mainstream population.
For more information see the following sites, which all have different philosophical viewpoints on FTM: