This discussion will focus on factors that are internal to the acid which affect its strength.   Consider the following acids:

Acid	Name	pKa
A	Chloroacetic	2.85
B	Dichloroacetic	1.25
C	Trichloroacetic	0.66

The strength of acid increases with the addition of extra chlorine atoms.  This simply illustrates that as X^- becomes more electronegative, the anion becomes a more stable conjugate base as it can spread the charge better, and hence its corresponding acid is stronger.  The following set of four acids also illustrates this point:

Acid	Name	pKa
D	Acetic	4.76
E	Iodoacetic	3.16
F	Bromoacetic	2.9
A	Chloroacetic	2.85

This set shows that the electronegativity of the substituents is in the order: H<I<Br<Cl.

Alkyl chains are slightly electron donating with respect to hydrogen.  Therefore, their presence will increase the charge density on the carboxylate ion, thus destabilising it.  The effect can be seen in the following:

Acid	Name	pKa
G	Methanoic	3.75
D	Acetic	4.76
H	Octanoic	4.90

The effect of increasing the length of the alkyl chain is to reduce the strength of the acid, however, the effect is not marked for increasing beyond a chain of two.