The key difference between Advaita and Visishtadvaita is the latter's teaching that the Brahman has special attributes compared to the Atman. What are these attributes? Are there any other differences between the two philosophies?
You could write a book on the subject – indeed, several people have. Regarding the attributes of Brahman, perhaps the best you could do is excerpt some of the numerous definitions provided by Rāmānuja himself. One such definition can be found in Śrībhāṣya 1.1.1:
By the word "Brahman" is denoted the Supreme Person [Puruṣottama], who is by inherent nature [svabhāvataḥ] free from all imperfections [doṣa] and possesses hosts of auspicious qualities [kalyāṇaguṇa] which are countless and of matchless excellence [anavadhikātiśaya]. In all contexts the term "Brahman" is applied to whatever possesses the quality of greatness [bṛhattva], but its primary and most significant meaning is that Being whose greatness is of matchless excellence, both in His essential nature and in His other qualities. It is only the Lord of all [Sarveśvara] who is such a Being. Therefore the word "Brahman" is primarily used only to signify him.
(John Braisted Carman, trans.)
Another definitional passage can be found at Bhagavadgītābhāṣya 18.42:
भगवान्पुरुषोत्तमो वासुदेवः परब्रह्मशब्दाभिदेयो निरस्तनिखिलदोषगन्धः स्वाभाविकानवधिकातिशयज्ञानशक्त्याद्यसङ्ख्येयकल्याणगुणगणो निखिलवेदवेदान्तवेद्यः । स एव निखिलजगदेककारणं निखिलजगदाधारभूतः । निखिलस्य स एव प्रवर्तयिता । तदाराधनभूतं च वैदिकं कृत्स्नं कर्म । तैस्तैराराधितो धर्मार्थकाममोक्षाख्यं फलं प्रयच्छतीत्यस्यार्थस्य सत्यतानिश्चय आस्तिक्यम् ।
That which is expressed with the term "Supreme Brahman" is the Lord [Bhagavān], the Supreme Person [Puruṣottama] and Vāsudeva [Krishna]. He is removed beyond any trace [literally "scent"] of evil. He possesses a host of auspicious qualities such as knowledge and power, which are natural to Him and of matchless excellence. He is the one who is to be known through all the Vedas and Upaniṣads. He is the sole cause of the entire universe and is its support. He causes all things ot function. All duties enjoined by the Vedas constitute the worship of Him. Being worshipped through these various duties He grants, according to the petition, the fruit of the nature of dharma, artha, kāma and mokṣa.
(John Braisted Carman, trans.)
Another pithier summary can be found in the dedicatory verse (maṅgalaśloka) of the Vedāntadīpa:
श्रीमते निर्मलानन्दोदन्वते विष्णवे नम: ॥
I bow unto Viṣṇu, who has as his body all the sentient and non-sentient beings, who is the self of all objects, who is associated with Śrī and who is the ocean of bliss untainted with impurity.
(M.B. Narasimha Ayyangar, trans.)
You can see here what you might call the central doctrine of Viśiṣṭādvaita: all objects in the universe stand in the same relation to Brahman as the body does to the soul. A similar point is made in the maṅgalaśloka of the Vedārthasaṃgraha, which Keshav linked in a comment above.
As for differences with Advaita, perhaps the biggest is that Rāmānuja strongly defends the reality of the individual jīvātma, and a theistic (or maybe more precisely, panentheistic) conception of Brahman. A full exposition would take a lot of space, but in Vedārthasaṃgraha para. 81 we find:
इत्यादिवेदविदग्रेसरवालमोकि - पराशर – द्वैपायनवचोभिश्च, परस्य ब्रह्मणः सर्वस्य आत्मत्वावगमात्, चिदचिदात्मकस्य वस्तुनः तच्छरिरत्वावगमाच्च, शरीरस्य च शरीरिणं प्रति प्रकारतयैव पआर्थत्वात्, शरीरशरीरिणोश्च धर्मभेदेऽपि तयोरसङ्करात्, सर्वशरीरं ब्रह्मेति ब्रह्मणो वैभवं प्रतिपादयभ्दिः सामानाधिकरण्यादिभिः सुख्यवृत्तैः सर्वचेतनाचेतप्रकारं ब्रह्मैव अभिधीयते ।
Such are the words of the sages, like Vālmīki, Parāśara and Dvaipāyana, who are the best among the knowers of the Vedas. From both the Vedas and the words of the sages, emerges the following teaching: The supreme Brahman is the self of all. The sentient and non-sentient entities constitute its body. The body is an entity and has being only by virtue of its being the mode of the soul of which it is the body. The body and soul, though characterized by different attributes, do not get mixed up. From all this follows the central teaching that Brahman, with all the non-sentient and sentient entities as its modes [prakāras], is the ultimate. The scriptures declare this glory of Brahman by saying that Brahman has the whole universe as its body. They also identify Brahman and the world in the manner of co-ordinate predication, which bears in this connection direct and primary meaning.
(S.S. Raghavachar, trans.)
And in paragraph 117:
एवं च सति, अभेदो वा भेदो वा द्व्यात्मकता व वेदान्तवेद्यः कोऽयमर्थः समर्थितो भवति? सर्वस्य वेदवेद्यत्वात् सर्वं समर्थितम् । सर्वशरीतया सर्वप्रकारं ब्रह्मैव अवस्थितमिति, अभेदः समर्थितः; एकमेव ब्रह्म नानाभूतचिदचिद्वस्तुप्रकारं नानात्वे अवस्थितम् इति भेदाभेदौ; अचिद्वस्तुनश्च ईश्वरस्य च स्वरूपस्वभाववैलक्षण्यात् असङ्कराच्च भेदः समर्थितः
It may be asked, 'What is your final position? Do you uphold unity or plurality or both unity and plurality? Which of these three forms the substance of the Vedānta on your interpretation?' We reply that we uphold all the three as they are all affirmed in the Veda. We uphold unity because Brahman alone exists, with all other entities as its modes. We uphold both unity and plurality, as the one Brahman itself has all the spiritual and physical substances as its modes and thus exists qualified by a plurality. We uphold plurality as the three categories, sentient selves and non-sentient existents and the Supreme Lord, are mutually distinct in their substantive nature and attributes and there is no mutual transpostion of their characteristics.
(S.S. Raghavachar, trans.)
If you're looking for further reading in English online, you can find (Rāmānuja's characterization of) the Advaita position in the Great Pūrvapakṣa of his Śrībhāṣya, followed by Rāmānuja's rejoinder. The Internet Encyclopedia of Philosophy also has a short summary of Rāmānuja's position. Plus if you can make your question more specific, I can try and give you a more focused answer.