I get a lot of questions from readers, and most of the time, the answers are fairly short. When I feel the question or the reply would be valuable to others as well, I make a post with a collection of them and post them in one go. Today is one of those posts, even when the answers weren't short. Because they could sort of be grouped together as 'Hellenic Gods 101'.

"Can you tell me where can I find a list of old Greek Gods and/or a brief detail regarding them. Kind of a family tree. I found a number of them of internet but not sure which one is correct."

The funny part about genealogies of the Gods is that they are all equally 'correct', as long as they stem from ancient sources. Genealogies of the Gods were all written down by humans about the Gods, and there are a variety of them. Which one is entirely true is unknown, and perhaps none of them are--or all of them. The most famous account of how the Gods came to be comes from Hesiod. His 'Theogogy' is a complete recounting of the story, starting with Khaos:

"Verily at the first Chaos came to be, but next wide-bosomed Earth, the ever-sure foundations of all  the deathless ones who hold the peaks of snowy Olympus, and dim Tartarus in the depth of the wide-pathed Earth, and Eros, fairest among the deathless gods, who unnerves the limbs and overcomes the mind and wise counsels of all gods and all men within them. From Chaos came forth Erebus and black Night; but of Night were born Aether and Day, whom she conceived and bare from union in love with Erebus. And Earth first bare starry Heaven, equal to herself, to cover her on every side, and to be an ever-sure abiding-place for the blessed gods. And she brought forth long Hills, graceful haunts of the goddess-Nymphs who dwell amongst the glens of the hills. She bare also the fruitless deep with his raging swell, Pontus, without sweet union of love. But afterwards she lay with Heaven and bare deep-swirling Oceanus, Coeus and Crius and Hyperion and Iapetus, Theia and Rhea, Themis and Mnemosyne and gold-crowned Phoebe and lovely Tethys. After them was born Cronos the wily, youngest and most terrible of her children, and he hated his lusty sire." [ll. 116-138]

He goes on to list a great many deities, cutting out a rough shape of the cosmos while doing so. There are many variations of this family tree, and in the ancient writings, there are also creation stories that range beyond this basic framework. In the Orphic Theogonies, for example, the universe does not start with Khaos, but with Thesis and Hydros, reaching back to Hómēros:

"Originally there was Hydros (Water), he [Orpheus] says, and Mud, from which Ge (the Earth) solidified: he posits these two as first principles, water and earth . . . The one before the two [Thesis], however, he leaves unexpressed, his very silence being an intimation of its ineffable nature. The third principle after the two was engendered by these--Ge (Earth) and Hydros (Water), that is--and was a Serpent (Drakon) with extra heads growing upon it of a bull and a lion, and a god’s countenance in the middle; it had wings upon its shoulders, and its name was Khronos (Unaging Time) and also Herakles. United with it was Ananke (Inevitability, Compulsion) , being of the same nature, or Adrastea, incorporeal, her arms extended throughout the universe and touching its extremities. I think this stands for the third principle, occuping the place of essence, only he [Orpheus] made it bisexual [as Phanes] to symbolize the universal generative cause." [Theogonies Fragment 54] 
So I can't point you to one account. At best, I can tell you that many of the ancient writers had their own thoughts on the subject, undoubtedly inspired by the community they lived in, the region they lived in, and the circles they moved in.
"Is it true that Greek and Roman gods are/were the same, and that only their names are different?"
I am distinctly not of this opinion, no. Personally, I think that the Hellenic and Roman deities share the same (Hellenic) base, but that the Roman deities differ from the Hellenic ones. Some not so much, other a great deal. In general, I regard the Roman Gods as epithets of the Hellenic ones, with a few notable exceptions--especially where there is no viable counterpart in Hellenic mythology. Why? Well, for one  the Theoi came first. The Roman empire came up about a thousand years after the rise of the Theoi.  Hellenic mythology featured the Hellenes, their stories and their cities, while Roman mythology focussed on the Roman people, their stories and their cities. The Hellenes had the Iliad as a major introductory and poetic text to introduce the Theoi, and the Romans had their own text: the Aeneid, a Latin epic poem, written by Virgil between 29 and 19 BC, that tells the legendary story of Aeneas, a Trojan who travelled to Italy, where he became the ancestor of the Romans.

Differences in the two societies also reflected on the Gods and Their importance. For one, the Hellenes valued  physical prowess, but it were poets and scholars who were held in the highest regards. For Rome, it were the warriors who received the most attention. This reflected in the Gods of both people as well: the Roman Gods resemble the Hellenic Gods, but they are stricter, harder and possess more bloodlust. At the same time, they were also pruder when it came to excesses of any kind. Ares, temperamental God of War, has his Roman counterpart in Mars, yet, Mars is a much stabler God, who is also in charge of agriculture and fertility. Bacchus, the Roman equivalent of Dionysos, lost all ecstatic rites that made the worship of Dionysos so famous.

Another major example of the differences between the two religions was that the Romans had no set shape for their Gods: they looked different to every individual. They were not revered for Their beauty, like their Hellenic counterparts. The Hellenes knew exactly how their Gods looked. They were often described as having muscular bodies (for the men), beautiful eyes and hair (both men and women), and delicate ankles (women). They were role-models to strive towards. Not so for the Romans.

The Roman culture also had a thing for the afterlife. Where the Hellenes focussed on this life and saw death as an inevitable conclusion of it, the Romans struggled to do good deeds and live good lives to be rewarded in the afterlife. They felt that, if they had been good enough, brave enough, warrior-like enough, they would take their place with the Gods after death. The Hellenes worried more about the judgement of the Theoi while they were still alive and knew they would go to the Underworld afterwards. Of course, things changed in that regard already: the mysteries brought the idea of awareness after reincarnation, and parts of the Underworld fell into disuse.

It seems to me, that the Romans tried becoming Gods their whole lives, while the Hellenes accepted their lot as mortals, and respected the Theoi as all-powerful and all-ruling. A frame of mind like that shows in Gods that get neatly packaged, made non-threatening and can be rivalled by mortals. Yet, because of the warrior mentality of the Romans, the Gods that became more predictable and less formed, also became harder. They still punished socially unacceptable behaviour, however, and myths from the Hellenic period got retold from the viewpoint of a warrior's society.