Quick explanation:
Cognates are words in English and Spanish that sound very similar. They can be nouns, adjectives, verbs, adverbs, etc.
Let´s see a few examples:

el doctor - doctor
el animal, los animales - animal, animals.
activo - active
operar - to operate
exportar - to export
exacto - exact, accurate

Long Explanation:

NOUNS
Many English nouns can be converted into spanish nouns depending on its ending.

ending in "or" -commonly identical.

actor - actor	color - color
doctor - doctor	director - director
horror - horror

ending in "al" -commonly identical.

animal - animal	general - general
hospital - hospital	manual - manual
moral - moral

ending in "ist" -change to "ista".

turista - tourist	socialista - socialist
egoísta - egoist	artista - artist
lista - list

ending in "ism" -change to "ismo".

optimismo - optimism	organismo - organism
mecanismo - mechanism	despotismo - despotism
idealismo - idealism

ending in "nce" -change to encia.

inteligencia - intelligence	paciencia - patience
experiencia - experience	arrogancia - arrogance
abundancia - abundance

ending in "ty" -change to "dad".

electricidad - electricity	universidad - university
identidad - identity	actividad - activity
curiosidad - curiosity

ending in "tion" -change to "ción".

acción - action	instrucción - instruction
condición - condition	combinación - combination
asociación - association

ending in "y" -change to "ia", "ía", or "io".

diccionario - dictionary	testimonio - testimony
salario - salary	agonía - agony
familia - family	historia - history
batería - battery	urgencia - urgency

ADJECTIVES
Many English adjectives can also be converted to Spanish adjectives by changing the ending.

ending in "ive" -change to "ivo".

efectivo - effective	festivo - festive
indicativo - indicative	activo - active
comunicativo - communicative

ending in "al" -commonly identical.

criminal - criminal	natural - natural
usual - usual	brutal - brutal
local - local

ending in "ous" -change to "oso".

venenoso - venomous	generoso - generous
delicioso - delicious	gracioso - gracious
ambicioso - ambitious

ending in "ic" -change to "ico".

energético - nergetic	magnético - magnetic
fantástico - fantastic	automático - automatic
artístico - artistic

ending in "ble" -identical, but they change in pronunciation.

notable - notable	favorable - favourable
terrible - terrible	horrible - horrible
imposible - impossible

ending in "nt" -change to "nte".

importante - important	conveniente - convenient
triunfante - triumphant	ignorante - ignorant
prudente - prudent

ending in "id" -change to "ido".

sólido - solid	cándido - candid
espléndido - splendid	lúcido - lucid
estúpido - stupid

ending in "ile" -change to "il".

automóvil - automobile	juvenil - juvenile
frágil - fragile	ágil - agile

ending in "ary" -change to "ario".

imaginario - immaginary	ordinario - ordinary
arbitrario - arbitrary	actuario - actuary

VERBS
Many English verbs can be converted into Spanish by changing the ending in English and adding the Spanish verb suffices "ar", "er", or "ir".

ending in "ate" -change to "ar".

terminar - terminate	participar - participate
negociar - negotiate	exagerar - exaggerate
demostrar - demonstrate	acentuar - accentuate

ending in vowel+consonat+"t" -change to "ar" or "ir".

exportar - export	importar - import
insertar - insert	exaltar - exalt
convertir - convert	insistir - insist

ending in vowel+consonant+"e" -drop "e" and add "ar".

utilizar - utilize	imaginar - imagine
invitar - invite	escapar - escape
ignorar - ignore	analizar - analyze

ending in "ify" -change to "ificar".

verificar - verify	pacificar - pacify
justificar - justify	modificar - modify
unificar - unify	simplificar - simplify

ADVERBS

ending in "ly" -change to "mente".

difícilmente - difficultly	exactamente - exactly
naturalmente - naturally	finalmente - finally
normalmente - normally	absolutamente - absolutely